Restore correct description of gist command

Hi Pry developers,

I noticed while looking at the Pry help that the gist command seemed to have the same description as the playback command:
[1] pry(main)> help
...
  play               Playback a string variable or a method or a file as input.
...
  gist               Playback a string variable or a method or a file as input.

I tracked that down to https://github.com/amonat/pry/commit/b6b78157a5b502a2eaa0b997e98184dc0547895b, which re-implemented gist and seems to have accidentally included copying and pasting the description of the play command.

I restored the old description, and took the liberty of adding a period at the end, to match (most of) the other descriptions.

Thanks!