Go policy has been single space after periods in comments for some time.

The copyright header template at:

	https://golang.org/doc/contribute.html#copyright

also uses a single space.

Make them all consistent.

This CL was generated with:

	perl -i -npe 's,^(// Copyright [0-9]+ The Go Authors\.)  (All rights reserved\.)$,$1 $2,' $(git grep -l -E '^// Copyright [0-9]+ The Go Authors\.  All rights reserved\.$')

Follows https://golang.org/cl/20111.

Change-Id: I66671dddf821f5dc027bc254e0196b3e3a2bdf3b
Reviewed-on: https://go-review.googlesource.com/32878Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>