fix doctest

Author: jialuoo

bigframes/session/__init__.py

@@ -2064,6 +2064,7 @@ def read_gbq_function(
         note, row processor implies that the function has only one input
         parameter.
 
+            >>> import pandas as pd
             >>> @bpd.remote_function(cloud_function_service_account="default")
             ... def row_sum(s: pd.Series) -> float:
             ...     return s['a'] + s['b'] + s['c']