Broken docs link for thunar-volman in xfce4-about window

Hello!

On "About" dialog the link for Volume Manager (thunar-volman) is broken. Currently, it points to https://docs.xfce.org/xfce/thunar-volman/start

I think the page we are interested in is at https://docs.xfce.org/xfce/thunar/thunar-volman

Either we change the link for thunar-volman in "About" dialog or move the doc at the intended location.