Revert unnecessary change to chunksize docs.

Author: tswast

Diff for: pandas/core/frame.py

@@ -1125,7 +1125,7 @@ def to_gbq(self, destination_table, project_id=None, chunksize=None,
             the environment.
         chunksize : int, optional
             Number of rows to be inserted in each chunk from the dataframe.
-            Use ``None`` to load the dataframe in a single chunk.
+            Set to ``None`` to load the whole dataframe at once.
         reauth : bool, default False
             Force Google BigQuery to re-authenticate the user. This is useful
             if multiple accounts are used.