Changed manifesto link

This addresses <code>tests.test_mission.TestMission.test_major_link_destinations_are_correct</code> fail both on prod and stage 
http://qa-selenium.mv.mozilla.com:8080/view/Mozilla.com/job/mozilla.com.prod/lastCompletedBuild/testReport/tests.test_mission/TestMission/test_major_link_destinations_are_correct/