math/big: fix comment
math/big: fix comment

The variable is named b not bit.

R=golang-dev, agl
CC=golang-dev
https://codereview.appspot.com/7699043

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