Fix CategoricalDtype docstring (#14622)

This PR fixes the docstring in CategoricalDtype where the repr has added a new field (categories_dtype).

This PR fixes 2 doctest failures.

Author: galipremsagar

python/cudf/cudf/core/dtypes.py

@@ -228,11 +228,11 @@ def from_pandas(cls, dtype: pd.CategoricalDtype) -> "CategoricalDtype":
         >>> import pandas as pd
         >>> pd_dtype = pd.CategoricalDtype(categories=['b', 'a'], ordered=True)
         >>> pd_dtype
-        CategoricalDtype(categories=['b', 'a'], ordered=True)
+        CategoricalDtype(categories=['b', 'a'], ordered=True, categories_dtype=object)
         >>> cudf_dtype = cudf.CategoricalDtype.from_pandas(pd_dtype)
         >>> cudf_dtype
-        CategoricalDtype(categories=['b', 'a'], ordered=True)
-        """
+        CategoricalDtype(categories=['b', 'a'], ordered=True, categories_dtype=object)
+        """  # noqa: E501
         return CategoricalDtype(
             categories=dtype.categories, ordered=dtype.ordered
         )
@@ -246,10 +246,10 @@ def to_pandas(self) -> pd.CategoricalDtype:
         >>> import cudf
         >>> dtype = cudf.CategoricalDtype(categories=['b', 'a'], ordered=True)
         >>> dtype
-        CategoricalDtype(categories=['b', 'a'], ordered=True)
+        CategoricalDtype(categories=['b', 'a'], ordered=True, categories_dtype=object)
         >>> dtype.to_pandas()
-        CategoricalDtype(categories=['b', 'a'], ordered=True)
-        """
+        CategoricalDtype(categories=['b', 'a'], ordered=True, categories_dtype=object)
+        """  # noqa: E501
         if self._categories is None:
             categories = None
         else: