all: single space after period

Follow CL 20022 and consistently use single space after a period in
documentation.

Generated with:

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

Change-Id: Ia29ad823668f060e81293e848a79fc4b4857d94b
Reviewed-on: https://go-review.googlesource.com/73530
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_bsd.go

@@ -34,7 +34,7 @@ func Getgroups() (gids []int, err error) {
 		return nil, nil
 	}
 
-	// Sanity check group count.  Max is 16 on BSD.
+	// Sanity check group count. Max is 16 on BSD.
 	if n < 0 || n > 1000 {
 		return nil, EINVAL
 	}