Merge markup-error-7636: Fix markup error in docs/core/howtos/servers.rst

Author: khorn
Reviewers: Alex
Fixes: #7636

Fix a minor markup error in docs/core/howtos/servers.rst

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43072 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb