testing: fix testing docs

The Perm function return 0 or 1 or 2 or 3. 4 is not returned,
so that changed the argument to 5.

Change-Id: Ic980c71a9f29f522bdeef4fce70a6c2dd136d791
Reviewed-on: https://go-review.googlesource.com/c/go/+/209777Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -99,7 +99,7 @@
 // line order:
 //
 //     func ExamplePerm() {
-//         for _, value := range Perm(4) {
+//         for _, value := range Perm(5) {
 //             fmt.Println(value)
 //         }
 //         // Unordered output: 4