strings: add example for ContainsRune

Change-Id: I994f003c97a14d194df5f07dd217c0ff3b214741
Reviewed-on: https://go-review.googlesource.com/48874Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>

src/strings/example_test.go

@@ -47,6 +47,16 @@ func ExampleContainsAny() {
 	// false
 }
 
+func ExampleContainsRune() {
+	// Finds whether a string contains a particular Unicode code point.
+	// The code point for the lowercase letter "a", for example, is 97.
+	fmt.Println(strings.ContainsRune("aardvark", 97))
+	fmt.Println(strings.ContainsRune("timeout", 97))
+	// Output:
+	// true
+	// false
+}
+
 func ExampleCount() {
 	fmt.Println(strings.Count("cheese", "e"))
 	fmt.Println(strings.Count("five", "")) // before & after each rune