Fix links to the elixir docs

[nitsas]

Hi and thanks for all the Elixir examples! I started learning Elixir recently and they helped me get more familiar with it. ❤️

The issue:
There are 37 broken links towards the elixir docs. For example in post "String to list of single character strings" the link String.graphemes/1 doesn't work.

• current (broken) url: http://elixir-lang.org/docs/stable/elixir/String.html#graphemes/1
• correct url: https://hexdocs.pm/elixir/String.html#graphemes/1

They are fixed in (open) PR #14. Can you please merge it when you have the time?