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

bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh