Update README.md

SpringMT · web-flow · commit 93d601fbc98d · 2023-03-22T15:58:34.000+09:00
diff --git a/README.md b/README.md
@@ -1,5 +1,5 @@
 [![Gem Version](https://badge.fury.io/rb/zstd-ruby.svg)](https://badge.fury.io/rb/zstd-ruby)
-![Build Status](https://github.com/SpringMT/zstd-ruby/actions/workflows/ruby.yml/badge.svg?branch=master)
+![Build Status](https://github.com/SpringMT/zstd-ruby/actions/workflows/ruby.yml/badge.svg?branch=main)
 
 # zstd-ruby
 
