regexp: update comment on (*Regexp).Longest
regexp: update comment on (*Regexp).Longest

Missed this review comment.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7229084

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