[x/blog] go.blog: fix typo on constants blog post
[x/blog] go.blog: fix typo on constants blog post

LGTM=adg
R=adg, r
CC=golang-codereviews
https://golang.org/cl/133910043
X-Blog-Commit: 0f4f2b91480c6f15a10e5bc59b98d06a008b3056

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website