Update to String.to_integer and String.to_float

[trexnix]

Hi,

binary_to_integer and binary_to_float are undefined on Elixir v1.0.5.

I found that String.to_integer and String.to_float which respectively call :erlang.binary_to_integer and :erlang.binary_to_float under the hood do the work now.

Also fixed the wrong file inclusion in appendix-a.asciidoc.

Hope this helps!