adapted the docstring of xarray.DataArray.differentiate (#8127)

Co-authored-by: afisc <mail@alexfischer.eu>

Author: afisc

xarray/core/dataarray.py

@@ -5193,9 +5193,10 @@ def differentiate(
             The coordinate to be used to compute the gradient.
         edge_order : {1, 2}, default: 1
             N-th order accurate differences at the boundaries.
-        datetime_unit : {"Y", "M", "W", "D", "h", "m", "s", "ms", \
+        datetime_unit : {"W", "D", "h", "m", "s", "ms", \
                          "us", "ns", "ps", "fs", "as", None}, optional
-            Unit to compute gradient. Only valid for datetime coordinate.
+            Unit to compute gradient. Only valid for datetime coordinate. "Y" and "M" are not available as
+            datetime_unit.
 
         Returns
         -------