all: single space after period

The tree's pretty inconsistent about single space vs double space
after a period in documentation. Make it consistently a single space,
per earlier decisions, and changes in go repository. This means
contributors won't be confused by misleading precedence.

This CL was generated with:

	perl -i -npe 's,^(\s*// .+[a-z]\.)  +([A-Z]),$1 $2,' $(git grep -l -E '^\s*//(.+\.)  +([A-Z])')

on top of copyright headers change in https://golang.org/cl/32878.

Follows https://golang.org/cl/20022.

Change-Id: I821e4a300122b4668aa31e12eaa914db615e5369
Reviewed-on: https://go-review.googlesource.com/32879Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>