content: clarify slices code example

Giving the slice a slightly more descriptive name makes
the formatted string in the loop a bit less cryptic.

Change-Id: Idd17014d7528f133c747df080c59a440c533eb0f
Reviewed-on: https://go-review.googlesource.com/19402
Reviewed-by: Andrew Gerrand <adg@golang.org>