build: rename default branch from master to main

Author: LeelaPrasadMaturu

README.md

@@ -202,3 +202,4 @@ If you find a bug, or have a feature suggestion, please
 [issues]: https://github.com/googlemaps/google-maps-services-js/issues
 [contrib]: https://github.com/googlemaps/google-maps-services-js/blob/master/CONTRIBUTING.md
 [Maps JavaScript API]: https://developers.google.com/maps/documentation/javascript/
+