Bug 1500279 New accesskey for Check for Updates r=flod,Gijs

Differential Revision: https://phabricator.services.mozilla.com/D9415

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

toolkit/locales/en-US/toolkit/about/aboutAddons.ftl

@@ -140,7 +140,7 @@ detail-repository-value =
 
 detail-check-for-updates =
     .label = Check for Updates
-    .accesskey = F
+    .accesskey = U
     .tooltiptext = Check for updates for this add-on
 
 detail-show-preferences =