Fix documentation generation for equalize_hist

The current documentation (automatically generated by yard) lists an `equalize_hist` method in the search, but when you go to the actual method, it shows `-(cvmat)` as the method name: http://rubydoc.info/github/ruby-opencv/ruby-opencv/master/OpenCV/CvMat:equalize_hist.

The docs with the fix show the method name correctly: http://rubydoc.info/github/gonzedge/ruby-opencv/master/OpenCV/CvMat:equalize_hist.