Adjusting README links

Author: Theo Geddes

README.md

@@ -1,8 +1,7 @@
 # ESMValTool Tutorial
 
 [![Join the chat at https://gitter.im/ESMValGroup/Tutorial](
-https://badges.gitter.im/ESMValGroup/Tutorial.svg)](
-https://gitter.im/ESMValGroup/Tutorial?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+https://badges.gitter.im/ESMValGroup/Tutorial.svg)][gitter]
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3974592.svg)](
 https://doi.org/10.5281/zenodo.3974592)