Search for and parse only Erlang source files

[KornelH]

I've noticed the extension search for and try to parse source files with .erl ending, but as this pattern is regular expression, the dot can be any character, not necessarily a literal dot. For example run_erl, to_erl or start_erl scripts of Erlang/OTP. Let's require literal dot as proper file extension.

Additionally fix some typos around this function.