This should help make clear that Len is not counting runes.
Also delete empty string, which doesn't add much.

Change-Id: I1602352df1897fef6e855e9db0bababb8ab788ca
Reviewed-on: https://go-review.googlesource.com/78110
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>