Update WXPay.md

doc/WXPay.md

@@ -4,17 +4,16 @@
  
 ```dart
 import 'package:fluwx/fluwx.dart' as fluwx;
-fluwx.pay(WeChatPayModel(
-                  appId: 'wxd930ea5d5a258f4f', 
+fluwx.pay( appId: 'wxd930ea5d5a258f4f', 
                   partnerId: '1900000109',
                   prepayId: '1101000000140415649af9fc314aa427',
                   packageValue: 'Sign=WXPay',
                   nonceStr: '1101000000140429eb40476f8896f4c9',
                   timeStamp: '1398746574',
                   sign: '7FFECB600D7157C5AA49810D2D8F28BC2811827B',
-                  signType: '选填',
-                  extData: '选填'
-                ));
+                  signType: '$Optional}',
+                  extData: '$Optional}'
+                );
 ```
 ### Return 
 The return value of `fluwx.share(model)` is a `Map`：