[x/blog] go.blog/slices: make slice-header-modifying example actually modify it Fixes golang/go#6570.
[x/blog] 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
X-Blog-Commit: 190a69e0573a68c27a4cbda968fadf4eec3c1d60

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