Improvement of documentation of function File.rm_rf!

[RaymondLoranger]

Elixir and Erlang/OTP versions

The documentation is not a correct sentence.

Operating system

N/A

Current behavior

Same as rm_rf/1 but raises a File.Error exception in case of failures, otherwise the list of files or directories removed.

Expected behavior

Same as rm_rf/1 but raises a File.Error exception in case of failures, otherwise returns the list of files or directories removed.

[josevalim]

Going to be fixed in #14528. Thank you!