DOC: Add categorical column in example DataFrame

Closes pandas-dev#27955

Author: galuhsahid

doc/source/user_guide/io.rst

@@ -4724,7 +4724,8 @@ See the documentation for `pyarrow <https://arrow.apache.org/docs/python/>`__ an
                       'd': np.arange(4.0, 7.0, dtype='float64'),
                       'e': [True, False, True],
                       'f': pd.date_range('20130101', periods=3),
-                      'g': pd.date_range('20130101', periods=3, tz='US/Eastern')})
+                      'g': pd.date_range('20130101', periods=3, tz='US/Eastern'),
+                      'h': pd.Categorical(list('abc'))})
 
    df
    df.dtypes