doc: fix typos and trailing spaces
doc: fix typos and trailing spaces

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/8285044

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang