Updater: Append trailing slash to upgrade url

* It gets appended after calling anyway
* Having the url at the end of a linkified message with a dot after it
  (end of sentence, e.g. russian), the dot is also linkified resulting
  in
  https://wiki.lineageos.org/devices/<device>/upgrade.
  which won't work, while the slash makes it work
  (with the linkified dot)

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/7248
Change-Id: Iaacd61267ad886c23393e34071a191a2e08d4009