fix typo

Author: wenxiangjiang

README.md

@@ -33,15 +33,15 @@ To check the availability of Google Play services on the current device, you can
 ``` dart
 import `package:google_api_availability/google_api_availability.dart`;
 
-GooglePlayServicesAvailability availability = await GoogleApiAvailability().checkGooglePlayServicesAvailability();
+GooglePlayServicesAvailability availability = await GoogleApiAvailability.instance.checkGooglePlayServicesAvailability();
 ```
 
 If you want to show a dialog by the Android system with more information and show a possible fix to the user you can do:
 
 ``` dart
 import `package:google_api_availability/google_api_availability.dart`;
 
-GooglePlayServicesAvailability availability = await GoogleApiAvailability().checkGooglePlayServicesAvailability(true);
+GooglePlayServicesAvailability availability = await GoogleApiAvailability.instance.checkGooglePlayServicesAvailability(true);
 ```
 
 **Note that on iOS this will always return false!**