doc: fix typo in sharemem codewalk
doc: fix typo in sharemem codewalk

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/98460045

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc