Clarified "missing_also_known_as" (#13746)

Fixes the confusion mentioned in https://github.com/tootsuite/mastodon/issues/12216. Suggestion of this fix provided by https://github.com/tootsuite/mastodon/issues/12216#issuecomment-564918757.

config/locales/en.yml

@@ -906,7 +906,7 @@ en:
     cancelled_msg: Successfully cancelled the redirect.
     errors:
       already_moved: is the same account you have already moved to
-      missing_also_known_as: is not back-referencing this account
+      missing_also_known_as: is not an alias of this account
       move_to_self: cannot be current account
       not_found: could not be found
       on_cooldown: You are on cooldown