bytes: change ExampleReader_Len to use a non-ASCII string
bytes: change ExampleReader_Len to use a non-ASCII string

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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

CAFxX/go,golang/go,josharian/go,codestation/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,Cofyc/go,codestation/go,akutz/go,golang/go,Cofyc/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,Cofyc/go,nawawi/go,akutz/go,josharian/go,Cofyc/go,codestation/go,nawawi/go,Cofyc/go,nawawi/go,cloudflare/go,josharian/go,akutz/go,josharian/go,Cofyc/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,Cofyc/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,codestation/go,golang/go,codestation/go,akutz/go,golang/go,josharian/go,josharian/go,codestation/go,nawawi/go,golang/go,nawawi/go,akutz/go,josharian/go,Cofyc/go,christopher-henderson/Go,cloudflare/go,codestation/go,codestation/go,akutz/go,nawawi/go