go.blog: fix some minor typos in "Inside the Go Playground".
go.blog: fix some minor typos in "Inside the Go Playground".

R=adg, r
CC=golang-dev
https://codereview.appspot.com/40720051

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