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/id_token.py b/google/oauth2/id_token.py
index d70782b..5e36260 100644
--- a/google/oauth2/id_token.py
+++ b/google/oauth2/id_token.py
@@ -50,7 +50,7 @@
     cached_session = cachecontrol.CacheControl(session)
     request = google.auth.transport.requests.Request(session=cached_session)
 
-.. _OpenID Connect ID Token:
+.. _OpenID Connect ID Tokens:
     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
 .. _CacheControl: https://cachecontrol.readthedocs.io
 """