Merge process-howto-typo-7642: Fix typo in process howto.

Author: Alex
Reviewer: khorn
Fixes: #7642

This fixes a minor typo in the process howto, which was causing a link to be
rendered incorrectly by Sphinx.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43140 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb