docs: pinned the Contracts Testing badge to master

Author: jaybuidl

README.md

@@ -9,7 +9,7 @@
 </p>
 
 <p align="center">
-  <a href="https://github.com/kleros/kleros-v2/actions/workflows/contracts-testing.yml"><img src="https://github.com/kleros/kleros-v2/actions/workflows/contracts-testing.yml/badge.svg" alt="Unit testing"></a>
+  <a href="https://github.com/kleros/kleros-v2/actions/workflows/contracts-testing.yml"><img src="https://github.com/kleros/kleros-v2/actions/workflows/contracts-testing.yml/badge.svg?branch=master" alt="Unit testing"></a>
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   <a href="http://commitizen.github.io/cz-cli/"><img src="https://img.shields.io/badge/commitizen-friendly-brightgreen.svg" alt="Commitizen Friendly"></a>
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