build: fix warnings in docgen (#711)

Resolved all the outstanding warnings from sphinx.

Going forward, warnings will be treated as errors since the `-W` flag is being used with `sphinx-build` 
diff --git a/google/oauth2/utils.py b/google/oauth2/utils.py
index efda796..593f032 100644
--- a/google/oauth2/utils.py
+++ b/google/oauth2/utils.py
@@ -101,7 +101,7 @@
 
         Args:
             headers (Mapping[str, str]): The HTTP request header.
-            request_body (Optional[Mapping[str, str]): The HTTP request body
+            request_body (Optional[Mapping[str, str]]): The HTTP request body
                 dictionary. For requests that do not support request body, this
                 is None and will be ignored.
             bearer_token (Optional[str]): The optional bearer token.