Fix test.

Author: dcherian

xarray/core/groupby.py

@@ -400,7 +400,7 @@ def __init__(
         unique_coord: DataArray | IndexVariable | _DummyGroup
         if grouper is not None:
             index = safe_cast_to_index(group)
-            if not index.is_montonic_increasing:
+            if not index.is_monotonic_increasing:
                 # TODO: sort instead of raising an error
                 raise ValueError(
                     "Index must be monotonic and increasing for resampling."

xarray/tests/test_groupby.py

@@ -1504,7 +1504,7 @@ def test_resample(self):
         expected = DataArray.from_series(expected_)
         assert_identical(actual, expected)
 
-        with pytest.raises(ValueError, match=r"index must be monotonic"):
+        with pytest.raises(ValueError, match=r"Index must be monotonic"):
             array[[2, 0, 1]].resample(time="1D")
 
         reverse = array.isel(time=slice(-1, None, -1))