Fix some typos (#1595)

*Fix typos in decorators.py, sphdistance.py, and test_grdgradient.py

pygmt/helpers/decorators.py

@@ -173,9 +173,9 @@
             [**a**]\ **x**\|\ **y**\|\ **d**\|\ **X**\|\ **Y**\|\
             **D**\|[*col*]\ **z**\ *gap*\ [**+n**\|\ **p**].
             Examine the spacing between consecutive data points in order to
-            impose breaks in the line. To specify multiple critera, provide
+            impose breaks in the line. To specify multiple criteria, provide
             a list with each item containing a string describing one set of
-            critera. Prepend **a** to specify that all the criteria must be
+            criteria. Prepend **a** to specify that all the criteria must be
             met [Default is to impose breaks if any criteria are met]. The
             following modifiers are supported:
 

pygmt/src/sphdistance.py

@@ -74,7 +74,7 @@ def sphdistance(data=None, x=None, y=None, **kwargs):
         - **n** - assign the ID numbers of the Voronoi polygons that each
           grid node is inside
         - **z** - assign all nodes inside the polygon the z-value of the center
-          node fot a natural nearest-neighbor grid.
+          node for a natural nearest-neighbor grid.
 
         Optionally, append the resampling interval along Voronoi arcs in
         spherical degrees.

pygmt/tests/test_grdgradient.py

@@ -63,5 +63,5 @@ def test_grdgradient_fails(grid):
     with pytest.raises(GMTInvalidInput):
         grdgradient(grid=grid)  # fails without required arguments
     with pytest.raises(GMTInvalidInput):
-        # failes when tiles is specified but not normalize
+        # fails when tiles is specified but not normalize
         grdgradient(grid=grid, azimuth=10, direction="c", tiles="c")