README.md: fix codecov URL

Author: tgamblin

README.md

@@ -2,7 +2,7 @@
 ![linux](https://github.com/spack/sbang/workflows/linux/badge.svg)
 ![macos](https://github.com/spack/sbang/workflows/macos/badge.svg)
 ![shellcheck](https://github.com/spack/sbang/workflows/shellcheck/badge.svg)
-[![codecov](https://codecov.io/gh/spack/sbang/branch/main/graph/badge.svg?token=IKH7mB5qq7)](undefined)
+[![codecov](https://codecov.io/gh/spack/sbang/branch/main/graph/badge.svg?token=IKH7mB5qq7)](https://codecov.io/gh/spack/sbang)
 
 `sbang` lets you run scripts with very long shebang (`#!`) lines.