fix(i18n): use American spelling (#1751)

locales/en-GB.json

@@ -6,6 +6,9 @@
     "favourite": "Favourite",
     "favourited": "Favourited"
   },
+  "menu": {
+    "show_favourited_and_boosted_by": "Show who favourited and boosted"
+  },
   "nav": {
     "favourites": "Favourites"
   },

locales/en.json

@@ -215,7 +215,7 @@
     "open_in_original_site": "Open in original site",
     "pin_on_profile": "Pin on profile",
     "share_post": "Share this post",
-    "show_favourited_and_boosted_by": "Show who favourited and boosted",
+    "show_favourited_and_boosted_by": "Show who favorited and boosted",
     "show_reblogs": "Show boosts from {0}",
     "show_untranslated": "Show untranslated",
     "toggle_theme": {