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/service_account.py b/google/oauth2/service_account.py
index ed91011..1ccfa19 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -424,6 +424,7 @@
             service_account.IDTokenCredentials.from_service_account_file(
                 'service-account.json'))
 
+
     Or if you already have the service account file loaded::
 
         service_account_info = json.load(open('service_account.json'))
@@ -431,6 +432,7 @@
             service_account.IDTokenCredentials.from_service_account_info(
                 service_account_info))
 
+
     Both helper methods pass on arguments to the constructor, so you can
     specify additional scopes and a subject if necessary::
 
@@ -439,7 +441,8 @@
                 'service-account.json',
                 scopes=['email'],
                 subject='user@example.com'))
-`
+
+
     The credentials are considered immutable. If you want to modify the scopes
     or the subject used for delegation, use :meth:`with_scopes` or
     :meth:`with_subject`::