wrong typespec for ExDoc.Language.Elixir.module_data/3

[a-maze-d]

The ExDoc.Language.Elixir module defines a typespec for module_data/3 that is incorrect.

The language: is defined as ExDoc.Language.Erlang instead of ExDoc.Language.Elixir as returned by the implementation which sets the language: __MODULE__

I suspect this is a copy&paste error.

I'm currently working on something else, but I'm happy to provide a PR soon(ish)

[josevalim]

Please do PR!