Update undefined function error message in Getting Started Guide
Keep the pattern to place module names between `` chars
Reflect IO.puts argument to be an io_list to GS Guide

This requirement in IO.puts can be see in
10a795ff44129b8ac1f290b1a44c6e46cbf2bce3
Merge pull request #155 from Irio/small-improvements

Small improvements in texts of Getting Started Guide