image: fix typo in Rectangle.Sub comment.
image: fix typo in Rectangle.Sub comment.

Fixes issue 2724.

R=r, dsymonds
CC=golang-dev
http://codereview.appspot.com/5555043

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