Revert coveralls badge to master (#2610)

* Revert coveralls badge to `master`

* Fix actual link

README.md

@@ -2,7 +2,7 @@
 
 # Extension methods to fluently assert the outcome of .NET tests
 [![](https://img.shields.io/github/actions/workflow/status/fluentassertions/fluentassertions/build.yml?branch=develop)](https://github.com/fluentassertions/fluentassertions/actions?query=branch%3Adevelop)
-[![Coveralls branch](https://img.shields.io/coverallsCoverage/github/fluentassertions/fluentassertions?branch=develop)](https://coveralls.io/github/fluentassertions/fluentassertions?branch=develop)
+[![Coveralls branch](https://img.shields.io/coverallsCoverage/github/fluentassertions/fluentassertions?branch=master)](https://coveralls.io/github/fluentassertions/fluentassertions?branch=master)
 [![qodana](https://github.com/fluentassertions/fluentassertions/actions/workflows/code_quality.yml/badge.svg)](https://github.com/fluentassertions/fluentassertions/actions/workflows/code_quality.yml)
 [![](https://img.shields.io/github/release/FluentAssertions/FluentAssertions.svg?label=latest%20release&color=007edf)](https://github.com/FluentAssertions/FluentAssertions/releases/latest)
 [![](https://img.shields.io/nuget/dt/FluentAssertions.svg?label=downloads&color=007edf&logo=nuget)](https://www.nuget.org/packages/FluentAssertions)