labelbones: fix skeleton description

From Evan's bug report:

  There was one thing I should mention though, for what it's worth, to
  do with your labelbones.py script.
  https://github.com/rjw57/openni-skeleton-export/blob/master/examples/labelbones.py

  I had a lot of fun re-writing it in Matlab, and in the process found a
  bug on line 120:

  right_collar = (6,12) should be right_collar = (2,12)

  As it stands, the left_collar bone connects the base of the neck to
  the left shoulder, but the right_collar bone connects the right
  shoulder to the left shoulder. This is the cause for the noisy patch
  on the left_collar body-part in
  https://github.com/rjw57/openni-skeleton-export/blob/master/examples/img/labelbones.png