Quickstart updates

Most of these are minor typo fixes that I saw when working through the quick start guide. The two changes that I'm least comfortable with are the code updates and the ERL_LIB note. I noticed that the generated code and the messenger-back example had changed since the documentation was written. The main change was the switch from the (module:function args) form to (: module function args). Since both look like they work, I assumed that there must be a preference for the latter form. As for the ERL_LIB note, it really looked like this had been fixed. Please ignore these changes if I interpreted incorrectly.