docs: fix code block formatting in 'user-guide.rst' (#794)

docs/user-guide.rst

@@ -479,16 +479,16 @@ IDToken verification can be done for various type of IDTokens using the
 and ES256 algorithms. However, ES256 algorithm won't be available unless
 `cryptography` dependency of version at least 1.4.0 is installed. You can check
 the dependency with `pip freeze` or try `from google.auth.crypt import es256`.
-The following is an example of verifying ID tokens:
+The following is an example of verifying ID tokens ::
 
     from google.auth2 import id_token
 
     request = google.auth.transport.requests.Request()
 
     try:
-      decoded_token = id_token.verify_token(token_to_verify,request)
+        decoded_token = id_token.verify_token(token_to_verify,request)
     except ValueError:
-      # Verification failed.
+        # Verification failed.
 
 A sample end-to-end flow using an ID Token against a Cloud Run endpoint maybe ::