content: rename variable in nil slices example
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 <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

dupoxy/go-tour-fr,shuLhan/go-tour-id,Go-zh/tour,shuLhan/tour,golians/tour,zorion/go-tour-ca,scryner/tour,dupoxy/go-tour-fr,scryner/tour,golians/tour,Go-zh/tour,dupoxy/go-tour-fr,scryner/tour,shuLhan/tour,shuLhan/tour,golang/tour,zorion/go-tour-ca,scryner/tour,shuLhan/go-tour-id,Go-zh/tour,shuLhan/go-tour-id,Go-zh/tour,golians/tour