Updates Elixir gitignore file

Official `elixir`'s `gitignore` after running `mix new` contains these two lines:

> # Where 3rd-party dependencies like ExDoc output generated docs.
> /doc
> # Ignore .fetch files in case you like to edit your project deps locally.
> /.fetch

They are missing at the moment.

Elixir.gitignore

 /_build
 /cover
 /deps
+/doc
+/.fetch
 erl_crash.dump
 *.ez
 *.beam