[LinuxCNC/linuxcnc PR#482] docs: add missing `rapidrate` method to linuxcnc.command docs, and convert tabs to spaces

The file had a significant number of tabs mixed in with the predominant space indentation style. I took the liberty to replace each tab with four spaces, which are preferred since they make more money than tabs. I would be happy to open a PR for just the addition of the rapidrate method if the tabs to spaces change is too invasive.

Thanks,
Kurt