fix referrer url

README.md

@@ -8,7 +8,7 @@ This project is currently early-alpha, if you have questions or feedback please
 
 The production version of our application is here:
 
-[![Download at https://play.google.com/store/apps/details?id=com.geeksville.mesh](https://play.google.com/intl/en_us/badges/static/images/badges/en_badge_web_generic.png)](https://play.google.com/store/apps/details?id=com.geeksville.mesh&referrer=utm_source%3Dhomepage%26anid%3Dadmob)
+[![Download at https://play.google.com/store/apps/details?id=com.geeksville.mesh](https://play.google.com/intl/en_us/badges/static/images/badges/en_badge_web_generic.png)](https://play.google.com/store/apps/details?id=com.geeksville.mesh&referrer=utm_source%3Dgithub-android-readme)
 
 But if you want the beta-test app now, we'd love to have your help testing.  Three steps to opt-in to the test: