docs: See ofBackground is 404

[CarlFK]

this page: https://openframeworks.cc/documentation/gl/ofGLRenderer/#show_background

has a link:

See ofBackground

https://openframeworks.cc/documentation/ofGraphics/!#show_ofBackground 404

observation: https://openframeworks.cc/documentation/gl/ofGLRenderer/#show_background is not the same page as https://openframeworks.cc/documentation/gl/ofGLRenderer/ which bugs me a bit, but not sure this is something that needs to be fixxed.