let the user know what exactly was already tapped

Closes Homebrew/homebrew#44843.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Library/Homebrew/cmd/tap.rb

@@ -15,7 +15,7 @@ module Homebrew
     else
       user, repo = tap_args
       clone_target = ARGV.named[1]
-      opoo "Already tapped!" unless install_tap(user, repo, clone_target)
+      opoo "#{user}/#{repo} already tapped!" unless install_tap(user, repo, clone_target)
     end
   end