fmt: fix one-letter typo in doc.go Stupid mistake in previous CL.
fmt: fix one-letter typo in doc.go
Stupid mistake in previous CL.

TBR=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/166880043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go