Fix a typo [skip ci]

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: seisman

pygmt/tests/test_grdtrack.py

@@ -135,7 +135,7 @@ def test_grdtrack_wrong_kind_of_points_input(dataarray, dataframe):
 
 def test_grdtrack_wrong_kind_of_grid_input(dataarray, dataframe):
     """
-    Run grdtrack using grid input that is not as xarray.DataArray or file.
+    Run grdtrack using grid input that is not an xarray.DataArray or file.
     """
     invalid_grid = dataarray.to_dataset()
     with pytest.raises(GMTInvalidInput):