Use correct random choice function in code snippet

A hidden code snippet uses `tm.choice()` when it should use
`np.random.choice()`

Author: Chris Carroux <[email protected]>

Closes #12384 from carroux/patch-1 and squashes the following commits:

8babbdc [Chris Carroux] Use correct random choice function in code snippet

Author: carroux

doc/source/groupby.rst

@@ -217,7 +217,7 @@ the length of the ``groups`` dict, so it is largely just a convenience:
    weight = np.random.normal(166, 20, size=n)
    height = np.random.normal(60, 10, size=n)
    time = pd.date_range('1/1/2000', periods=n)
-   gender = tm.choice(['male', 'female'], size=n)
+   gender = np.random.choice(['male', 'female'], size=n)
    df = pd.DataFrame({'height': height, 'weight': weight,
                       'gender': gender}, index=time)