README: Fix coverage badge URL (#9046)

Author: RenjiSann

README.md

@@ -13,7 +13,7 @@
 [![License](http://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/uutils/coreutils/blob/main/LICENSE)
 [![dependency status](https://deps.rs/repo/github/uutils/coreutils/status.svg)](https://deps.rs/repo/github/uutils/coreutils)
 
-[![CodeCov](https://codecov.io/gh/uutils/coreutils/branch/master/graph/badge.svg)](https://codecov.io/gh/uutils/coreutils)
+[![CodeCov](https://codecov.io/gh/uutils/coreutils/branch/main/graph/badge.svg)](https://codecov.io/gh/uutils/coreutils)
 ![MSRV](https://img.shields.io/badge/MSRV-1.85.0-brightgreen)
 [![Weblate](https://hosted.weblate.org/widget/rust-coreutils/svg-badge.svg)](https://hosted.weblate.org/projects/rust-coreutils/)