seratch and bradfitz all: fix typos in comments
This pull request fixes several typos in comments. I believe that they're safe enough.

Change-Id: Ic7c594bdb89dcd04f8775f225abbc0f966f3728e
GitHub-Last-Rev: 2394feb
GitHub-Pull-Request: golang/tools#45
Reviewed-on: https://go-review.googlesource.com/128956
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Latest commit 4354ead Aug 10, 2018