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>
diff --git a/content/moretypes/nil-slices.go b/content/moretypes/nil-slices.go
index daa9ddc..a317027 100644
--- a/content/moretypes/nil-slices.go
+++ b/content/moretypes/nil-slices.go
@@ -5,9 +5,9 @@
 import "fmt"
 
 func main() {
-	var z []int
-	fmt.Println(z, len(z), cap(z))
-	if z == nil {
+	var s []int
+	fmt.Println(s, len(s), cap(s))
+	if s == nil {
 		fmt.Println("nil!")
 	}
 }