pick up esmvalcore badge instead of esmvaltool one in README (#1749)

pick uo core badge instead of tool badge

Author: valeriupredoi

README.md

@@ -1,6 +1,6 @@
 # ESMValCore package
 
-[![Documentation Status](https://readthedocs.org/projects/esmvaltool/badge/?version=latest)](https://esmvaltool.readthedocs.io/en/latest/?badge=latest)
+[![Documentation Status](https://readthedocs.org/projects/esmvalcore/badge/?version=latest)](https://esmvaltool.readthedocs.io/en/latest/?badge=latest)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3387139.svg)](https://doi.org/10.5281/zenodo.3387139)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/ESMValGroup?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![CircleCI](https://circleci.com/gh/ESMValGroup/ESMValCore/tree/main.svg?style=svg)](https://circleci.com/gh/ESMValGroup/ESMValCore/tree/main)