docs: exports. removed from the cloud function's entry point (#244)

Cloud function's entry point shouldn't be prefixed with `exports.`.

Co-authored-by: Gal Zahavi <38544478+galz10@users.noreply.github.com>

Author: Alexander Prikhodko

samples/webhooks.js

@@ -16,7 +16,7 @@
 
 // [START dialogflow_cx_webhook]
 
-// TODO: change entry point to exports.handleWebhook in cloud function
+// TODO: change entry point to handleWebhook in cloud function
 
 exports.handleWebhook = (request, response) => {
   const tag = request.body.fulfillmentInfo.tag;