Example Code typo fix.

`result` should change with each `SetState()` in `_onQRViewCreated(QRViewController controller))`, not `qrText`. `qrText` isn't defined!

README.md

@@ -119,7 +119,7 @@ class _QRViewExampleState extends State<QRViewExample> {
     this.controller = controller;
     controller.scannedDataStream.listen((scanData) {
       setState(() {
-        qrText = scanData;
+        result = scanData;
       });
     });
   }