go.blog/slices: make slice-header-modifying example actually modify it Fixes golang/go#6570.
go.blog/slices: make slice-header-modifying example actually modify it
Fixes golang/go#6570.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14614043

gorakhargosh/blog,gorakhargosh/blog,jeffreyliguori/blog