Fix http.testing docs example
Fixed syntax errors (two closing parentheses were missing) and return
type of function literal. The sample code was formatted with `dartfmt`.
diff --git a/lib/testing.dart b/lib/testing.dart
index f7fa3be..d5a7874 100644
--- a/lib/testing.dart
+++ b/lib/testing.dart
@@ -11,14 +11,15 @@
/// import 'dart:convert';
/// import 'package:http/testing.dart';
///
-/// var client = new MockClient((request) {
+/// var client = new MockClient((request) async {
/// if (request.url.path != "/data.json") {
/// return new Response("", 404);
/// }
-/// return new Response(JSON.encode({
-/// 'numbers': [1, 4, 15, 19, 214]
-/// }, 200, headers: {
-/// 'content-type': 'application/json'
-/// });
-/// };
+/// return new Response(
+/// JSON.encode({
+/// 'numbers': [1, 4, 15, 19, 214]
+/// }),
+/// 200,
+/// headers: {'content-type': 'application/json'});
+/// });
export 'src/mock_client.dart';
