chore: fix docstring typos (found via codespell) (googleapis#724)

```
codespell --version
2.1.0
```

google/cloud/ndb/_eventloop.py

@@ -246,7 +246,7 @@ def run_idle(self):
         """Run one of the idle callbacks.
 
         Returns:
-            bool: Indicates if an idle calback was called.
+            bool: Indicates if an idle callback was called.
         """
         if not self.idlers or self.inactive >= len(self.idlers):
             return False
@@ -270,7 +270,7 @@ def _run_current(self):
         """Run one current item.
 
         Returns:
-            bool: Indicates if an idle calback was called.
+            bool: Indicates if an idle callback was called.
         """
         if not self.current:
             return False

google/cloud/ndb/context.py

@@ -40,7 +40,7 @@ class _ContextIds:
     sufficient to uniquely identify the context in which a particular piece of code is
     being run. Each context, as it is created, is assigned the next id in this sequence.
     The context id is used by `utils.logging_debug` to grant insight into where a debug
-    logging statement is coming from in a cloud evironment.
+    logging statement is coming from in a cloud environment.
     """
 
     def __init__(self):
@@ -138,7 +138,7 @@ def get_toplevel_context(raise_context_error=True):
 
     The toplevel context is the context created by the call to
     :meth:`google.cloud.ndb.client.Client.context`. At times, this context will
-    be superceded by subcontexts, which are used, for example, during
+    be superseded by subcontexts, which are used, for example, during
     transactions. This function will always return the top level context
     regardless of whether one of these subcontexts is the current one.
 

google/cloud/ndb/global_cache.py

@@ -125,7 +125,7 @@ def set_if_not_exists(self, items, expires=None):
 
 
         Returns:
-            Dict[bytes, bool]: A `dict` mapping to boolean value wich will be
+            Dict[bytes, bool]: A `dict` mapping to boolean value that will be
                 :data:`True` if that key was set with a new value, and :data:`False`
                 otherwise.
         """

google/cloud/ndb/model.py

@@ -6156,7 +6156,7 @@ def _code_name_from_stored_name(cls, name):
         # class SomeKind(ndb.Model):
         #     foo = ndb.IntegerProperty(name="bar")
         #
-        # If we are passed "bar", we know to translate that to "foo", becasue
+        # If we are passed "bar", we know to translate that to "foo", because
         # the datastore property, "bar", is the NDB property, "foo". But if we
         # are passed "foo", here, then that must be the datastore property,
         # "foo", which isn't even mapped to anything in the NDB model.

tests/system/test_crud.py

@@ -1484,9 +1484,9 @@ class SomeKind(ndb.Model):
     key = entity.put()
     dispose_of(key._key)
 
-    retreived = key.get()
-    assert retreived.user.email() == "[email protected]"
-    assert retreived.user.auth_domain() == "gmail.com"
+    retrieved = key.get()
+    assert retrieved.user.email() == "[email protected]"
+    assert retrieved.user.auth_domain() == "gmail.com"
 
 
 @pytest.mark.usefixtures("client_context")
@@ -1508,9 +1508,9 @@ def user_id(self):
     key = entity.put()
     dispose_of(key._key)
 
-    retreived = key.get()
-    assert retreived.user.email() == "[email protected]"
-    assert retreived.user.auth_domain() == "gmail.com"
+    retrieved = key.get()
+    assert retrieved.user.email() == "[email protected]"
+    assert retrieved.user.auth_domain() == "gmail.com"
 
 
 @pytest.mark.usefixtures("client_context")
@@ -1527,8 +1527,8 @@ class SomeKind(ndb.Model):
     key = entity.put()
     dispose_of(key._key)
 
-    retreived = key.get()
-    assert retreived.foo == ["", ""]
+    retrieved = key.get()
+    assert retrieved.foo == ["", ""]
 
 
 @pytest.mark.skipif(not USE_REDIS_CACHE, reason="Redis is not configured")

tests/unit/orchestrate.py

@@ -219,7 +219,7 @@ def hither_and_yon(destination):
 class _Conductor:
     """Coordinate communication between main thread and a test thread.
 
-    Two way communicaton is maintained between the main thread and a test thread using
+    Two way communication is maintained between the main thread and a test thread using
     two synchronized queues (`queue.Queue`) each with a size of one.
     """