Fix mobile Maniphest Create link

Summary: Fixes T7367

Test Plan: I guess noone every used this? Click on mobile menu, get not a 404.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7367

Differential Revision: https://secure.phabricator.com/D11880