Change wording of "Manual Sync" button to clarify function

#641 (comment)

Author: mymindstorm

_locales/en/messages.json

@@ -282,8 +282,8 @@
         "description": "Edit"
     },
     "manual_dropbox": {
-        "message": "Manual Sync",
-        "description": "Manual sync"
+        "message": "Manual Backup",
+        "description": "Manual backup"
     },
     "use_autofill": {
         "message": "Use Autofill",