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>

rflanagan/reginaldflanagan-project1,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,mhennings/marcohennings-go,Triskite/willstone-goclone,Triskite/willstone-goclone,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,Triskite/willstone-goclone,Triskite/willstone-goclone,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,mhennings/marcohennings-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,webfd/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc