Updated Maturity level badge to new standard

PR Adapts the maturity level nomenclature to the new standard

Author: stefanoborini

README.md

@@ -2,7 +2,7 @@
 
 [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38d29fed78fa85652e3a63e154dd8e8829/media/badge.svg)](https://github.com/sindresorhus/awesome)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com)
-![Maturity level-0](https://img.shields.io/badge/Maturity%20Level-ML--0-red)
+![Maturity level-Prototype](https://img.shields.io/badge/Maturity%20Level-Prototype-red)
 
 A collection of research papers and software related to explainability in graph machine learning.