sort: update main example to use Slice along with Sort

sort: update main example to use Slice along with Sort

This is to let sort.Slice get more prominence since
it's the most common use case.

Fixes #21989

Change-Id: I0b180cc20256f5f09065b722e191c508c872f4f7
Reviewed-on: https://go-review.googlesource.com/65710
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>