image/color: rename modelYCbCr to yCbCrModel.
image/color: rename modelYCbCr to yCbCrModel.

This matches the other color models. It seems that this was missed
during the review of 5544073.

R=nigeltao, david.crawshaw
CC=golang-dev
http://codereview.appspot.com/5536057

Committer: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang