content: rename variable in nil slices example

The name z for a slice is a bit cryptic.

In retrospect it does make sense ("zero value of a slice"),
but a straight-forward s for slice seems better here.

Change-Id: I7afae523a8f567d3221299e4c0c9fa47789961cf
Reviewed-on: https://go-review.googlesource.com/19403
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed