README: Update Codecov badge and URL

- Display a badge for the main branch in the badge, as done for the
  other badges.
- Update the link to one that displays coverage information for the
  branch.

Signed-off-by: Kevin Locke <[email protected]>

Author: kevinoid

README.md

@@ -2,7 +2,7 @@
 ===============
 
 [![Build Status](https://img.shields.io/github/actions/workflow/status/kevinoid/git-branch-is/node.js.yml?branch=main&style=flat&label=build)](https://github.com/kevinoid/git-branch-is/actions?query=branch%3Amain)
-[![Coverage](https://img.shields.io/codecov/c/github/kevinoid/git-branch-is.svg?style=flat)](https://codecov.io/github/kevinoid/git-branch-is?branch=main)
+[![Coverage](https://img.shields.io/codecov/c/github/kevinoid/git-branch-is/main.svg?style=flat)](https://app.codecov.io/gh/kevinoid/git-branch-is/branch/main)
 [![Dependency Status](https://img.shields.io/david/kevinoid/git-branch-is.svg?style=flat)](https://david-dm.org/kevinoid/git-branch-is)
 [![Supported Node Version](https://img.shields.io/node/v/git-branch-is.svg?style=flat)](https://www.npmjs.com/package/git-branch-is)
 [![Version on NPM](https://img.shields.io/npm/v/git-branch-is.svg?style=flat)](https://www.npmjs.com/package/git-branch-is)