Make build badge working again

Signed-off-by: Mike Lischke <mike@lischke-online.de>

readme.md

@@ -1,4 +1,4 @@
-[![Build & Test](https://github.com/mike-lischke/jree/actions/workflows/nodejs.yml/badge.svg?branch=master)](https://github.com/mike-lischke/jree/actions/workflows/nodejs.yml) [![](https://img.shields.io/badge/java-11-4c7e9f.svg)](http://java.oracle.com) [![Downloads](https://img.shields.io/npm/dw/jree?color=blue)](https://www.npmjs.com/package/jree)
+[![Build & Test](https://github.com/mike-lischke/jree/actions/workflows/nodejs.yml/badge.svg?branch=main)](https://github.com/mike-lischke/jree/actions/workflows/nodejs.yml) [![](https://img.shields.io/badge/java-11-4c7e9f.svg)](http://java.oracle.com) [![Downloads](https://img.shields.io/npm/dw/jree?color=blue)](https://www.npmjs.com/package/jree)
 
 # Java Runtime Environment Emulation