Clarify tutorial description for endpoint creation

[Ben Pruce]

https://twilio-labs.github.io/socless/your-first-endpoint/
" At the bottom of the file, let’s configure a lambda function that:" - is a bit confusing when going through the demo and can lead to improperly updating your yml file. Probably makes sense to add a code block showing https://github.com/twilio-labs/socless-examples/blob/master/getting-started-tutorial/socless-tutorial/serverless.yml so its clear.