Correct an ambiguous regexp in DiffusionRequest

Summary:
See https://discourse.phabricator-community.org/t/diffusionrequest-regex-error/2057/.

The intent of [\d-,] is "digits, hyphen, and comma" but [x-y] means "character range x-y".

Specify [\d,-] instead to disambiguate the hyphen as "literal hyphen", not a character range marker.

Test Plan: I can't reproduce the original error as reported, but browsed around Diffusion for a bit.

Reviewers: amckinley, avivey

Reviewed By: avivey

Differential Revision: https://secure.phabricator.com/D19770