Fix wording of "differential.sticky-accept"

Summary: See <http://fab.wmflabs.org/T226>. The summary for this option is confusing, because "true" means sticky but the wording implies "true" means non-sticky.

Test Plan:
  - Looked at the option in summary view.
  - Reviewed related text, none of the other copy here seems confusing or ambiugous to me.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: qgil, epriestley

Differential Revision: https://secure.phabricator.com/D8884