Fix coveralls badge (#1906)

* Update README.md

* Use branch master for coveralls badge

README.md

@@ -3,7 +3,7 @@
 [![](https://img.shields.io/nuget/dt/FluentAssertions.svg?label=nuget%20downloads)](https://www.nuget.org/packages/FluentAssertions)
 [![](https://img.shields.io/librariesio/dependents/nuget/FluentAssertions.svg?label=dependent%20libraries)](https://libraries.io/nuget/FluentAssertions)
 ![](https://img.shields.io/badge/release%20strategy-githubflow-orange.svg)
-[![Coverage Status](https://coveralls.io/repos/github/fluentassertions/fluentassertions/badge.svg?branch=develop)](https://coveralls.io/github/fluentassertions/fluentassertions?branch=develop)
+[![Coverage Status](https://coveralls.io/repos/github/fluentassertions/fluentassertions/badge.svg?branch=master)](https://coveralls.io/github/fluentassertions/fluentassertions?branch=master)
 
 # About this project
 A very extensive set of extension methods that allow you to more naturally specify the expected outcome of a TDD or BDD-style unit tests. Targets .NET Framework 4.7, as well as .NET Core 2.1, .NET Core 3.0, .NET 6, .NET Standard 2.0 and 2.1.