README: fix coverage branch

Signed-off-by: Brian McGee <[email protected]>

README.md

@@ -9,7 +9,7 @@
 _A <a href="https://numtide.com/">numtide</a> project._
 
 <p>
-<a href='https://coveralls.io/github/numtide/treefmt?branch=new-README'><img src='https://coveralls.io/repos/github/numtide/treefmt/badge.svg?branch=new-README' alt='Coverage Status' /></a>
+<a href='https://coveralls.io/github/numtide/treefmt?branch=main'><img src='https://coveralls.io/repos/github/numtide/treefmt/badge.svg?branch=main' alt='Coverage Status' /></a>
 <a href="https://github.com/numtide/treefmt/actions/workflows/release.yml"><img src="https://github.com/numtide/treefmt/actions/workflows/release.yml/badge.svg"/></a>
 <img alt="Static Badge" src="https://img.shields.io/badge/status-beta-yellow">
 <a href="https://app.element.io/#/room/#home:numtide.com"><img src="https://img.shields.io/badge/Support-%23numtide-blue"/></a>