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>
diff --git a/unix/syscall_darwin_arm.go b/unix/syscall_darwin_arm.go
index d6d5573..47ab664 100644
--- a/unix/syscall_darwin_arm.go
+++ b/unix/syscall_darwin_arm.go
@@ -20,7 +20,7 @@
 //sysnb	gettimeofday(tp *Timeval) (sec int32, usec int32, err error)
 func Gettimeofday(tv *Timeval) (err error) {
 	// The tv passed to gettimeofday must be non-nil
-	// but is otherwise unused.  The answers come back
+	// but is otherwise unused. The answers come back
 	// in the two registers.
 	sec, usec, err := gettimeofday(tv)
 	tv.Sec = int32(sec)