caveats: Remove extra blank line between keg_only and other caveats

- This was pointed out by Carlo in
  https://github.com/Homebrew/brew/issues/14925#issuecomment-1473303922,
  it annoyed me the moment I saw it too, and it was a very quick fix.
- The caveats items are joined by "\n" before being displayed, so the
  extra `\n` here when generating the `keg_only_text` meant that there
  were two blank lines which looked weird. This `keg_only_text` method
  is only used here so it doesn't affect anywhere else we show the
  `keg_only` message, if there are any other places.

Library/Homebrew/caveats.rb

@@ -94,7 +94,7 @@ class Caveats
         end
       end
     end
-    s << "\n"
+    s
   end
 
   private