Bug 1581141 - Add access key to Turn on Sync. r=vladikoff,fluent-reviewers,flod

Differential Revision: https://phabricator.services.mozilla.com/D45859

--HG--
extra : moz-landing-system : lando

browser/locales/en-US/browser/menubar.ftl

@@ -213,6 +213,7 @@ menu-tools-sync-sign-in =
     .accesskey = y
 menu-tools-turn-on-sync =
     .label = Turn on { -sync-brand-short-name }…
+    .accesskey = n
 menu-tools-sync-now =
     .label = Sync Now
     .accesskey = S