I'm curious as to how you ended up with /forum/forums though? I'm guessing you uploaded the "forums" directory into your already existing "forum" one? I'm pretty picky with how everything looks and while adding a route filter to remove /forums might be the best and easiest way, if you only just installed the forum and have nothing on it, you can consider deleting the entire directory "/forums" and simply uploading the forum files (not "forums" folder) into your existing forum directory instead.
This basically means you'd be re-installing the forum again but it would permanently be in the first /forum directory if you did. It's really up to you how you go about it, I'm just picky like that.
If you do this: Remember to re-create the Database too.