Blackbox link 404

Created by: JayAndCatchFire

The current link to blackbox window manager, http://blackboxwm.sourceforge.net/, is dead.

I would have just made a PR to change it, but I'm not sure which link is preferred. Although the Sourceforge site is gone, the project is still there at https://sourceforge.net/projects/blackboxwm/. However, that page has a link saying the project was moved to https://github.com/bradleythughes/blackbox. That github project hasn't seen any activity in almost 3 years, though.