math/rand: add example for Intn func

Change-Id: I831ffb5c3fa2872d71def8d8461f0adbd4ae2c1a
GitHub-Last-Rev: 2adfcd2d5a
GitHub-Pull-Request: golang/go#30706
Reviewed-on: https://go-review.googlesource.com/c/go/+/166426
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/rand/example_test.go

@@ -140,3 +140,18 @@ func ExampleShuffle_slicesInUnison() {
 	// E: 5
 	// B: 2
 }
+
+func ExampleIntn() {
+	// Seeding with the same value results in the same random sequence each run.
+	// For different numbers, seed with a different value, such as
+	// time.Now().UnixNano(), which yields a constantly-changing number.
+	rand.Seed(86)
+	fmt.Println(rand.Intn(100))
+	fmt.Println(rand.Intn(100))
+	fmt.Println(rand.Intn(100))
+
+	// Output:
+	// 42
+	// 76
+	// 30
+}