Update client documentation (#91)

docs/usage/client.md

@@ -1,24 +1,23 @@
-A client connects their `YDoc` through a [WebsocketProvider](../reference/WebSocket_provider.md).
+A client connects their `Doc` through a [WebsocketProvider](../reference/WebSocket_provider.md).
 
 Here is a code example using the [websockets](https://websockets.readthedocs.io) library:
 ```py
 import asyncio
-import y_py as Y
 from websockets import connect
+from pycrdt import Doc, Map
 from pycrdt_websocket import WebsocketProvider
 
 async def client():
-    ydoc = Y.YDoc()
+    ydoc = Doc()
+    ymap = ydoc.get("map", type=Map)
     async with (
         connect("ws://localhost:1234/my-roomname") as websocket,
         WebsocketProvider(ydoc, websocket),
     ):
         # Changes to remote ydoc are applied to local ydoc.
         # Changes to local ydoc are sent over the WebSocket and
         # broadcast to all clients.
-        ymap = ydoc.get_map("map")
-        with ydoc.begin_transaction() as t:
-            ymap.set(t, "key", "value")
+        ymap["key"] = "value"
 
         await asyncio.Future()  # run forever