encoding/json: add example to Valid

Change-Id: I411483d76a2ca91cd15ff42ae1adb9134486d183
Reviewed-on: https://go-review.googlesource.com/c/145278Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/example_test.go

@@ -292,3 +292,12 @@ func ExampleMarshalIndent() {
 	// <prefix><indent>"b": 2
 	// <prefix>}
 }
+
+func ExampleValid() {
+	goodJSON := `{"example": 1}`
+	badJSON := `{"example":2:]}}`
+
+	fmt.Println(json.Valid([]byte(goodJSON)), json.Valid([]byte(badJSON)))
+	// Output:
+	// true false
+}