math/rand: fix typo in example comment.
math/rand: fix typo in example comment.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/6901056

tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go