math: add function examples.

The function Modf lacks corresponding examples.

Change-Id: Id93423500e87d35b0b6870882be1698b304797ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/231097
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/example_test.go

@@ -227,3 +227,14 @@ func ExampleCbrt() {
 	// 2.00
 	// 3.00
 }
+
+func ExampleModf() {
+	int, frac := math.Modf(3.14)
+	fmt.Printf("%.2f, %.2f\n", int, frac)
+
+	int, frac = math.Modf(-2.71)
+	fmt.Printf("%.2f, %.2f\n", int, frac)
+	// Output:
+	// 3.00, 0.14
+	// -2.00, -0.71
+}