time: fix docs for new comma layouts

The current text is slightly inaccurate. Make it more correct.

Change-Id: Iebe0051b74649d13982d7eefe3697f9e69c9b75d
Reviewed-on: https://go-review.googlesource.com/c/go/+/340449


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>