Take main→master rename into account

Author: tbroyer

README.md

@@ -51,5 +51,5 @@ Documentation
 
 The plugin documentation is published at https://tbroyer.github.io/gwt-maven-plugin/
 
-You can also look at the [integration tests](https://github.com/tbroyer/gwt-maven-plugin/tree/master/src/it),
+You can also look at the [integration tests](https://github.com/tbroyer/gwt-maven-plugin/tree/main/src/it),
 particularly `gwt-lib`, `gwt-app`, and `e2e`.