Ensures String.slice returns a string.

This is almost the same pull request as christhekeele's request here for issue #2412 :

https://github.com/elixir-lang/elixir/pull/2412

Except it corrects two typos in the docstring. Should I wait till christhekeele's request is merged and thenI make a separate PR with typos fixing only? I don't want to take credit for christhekeele's work.