Typo in documentation (#1787)

Wrong spelling of messages and single.

Co-authored-by: F. Hinkelmann <[email protected]>

appengine/pubsub/app.js

@@ -108,7 +108,7 @@ app.post('/pubsub/authenticated-push', jsonBodyParser, async (req, res) => {
     // Note: For high volume push requests, it would save some network
     // overhead if you verify the tokens offline by decoding them using
     // Google's Public Cert; caching already seen tokens works best when
-    // a large volume of messsages have prompted a singple push server to
+    // a large volume of messages have prompted a single push server to
     // handle them, in which case they would all share the same token for
     // a limited time window.
     const ticket = await authClient.verifyIdToken({