fmt: add an example for Sprintf

Signed-off-by: Venil Noronha <veniln@vmware.com>

Change-Id: Ie5f50bc31db1eee11582b70b0e25c726090d4037
Reviewed-on: https://go-review.googlesource.com/132236
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/example_test.go

@@ -27,3 +27,14 @@ func ExampleStringer() {
 	fmt.Println(a)
 	// Output: Gopher (2)
 }
+
+func ExampleSprintf() {
+	i := 30
+	s := "Aug"
+	sf := fmt.Sprintf("Today is %d %s", i, s)
+	fmt.Println(sf)
+	fmt.Println(len(sf))
+	// Output:
+	// Today is 30 Aug
+	// 15
+}