Change merge language in maniphest

Summary: Fixes T3467, changes "Merge Duplicates" to "Merge Duplicates In"

Test Plan: Reload Maniphest Page

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3467

Differential Revision: https://secure.phabricator.com/D6483