Update pip install command for tm-python

Author: arnobaer

README.md

@@ -11,5 +11,5 @@ It is recommended to install the utm Python bindings in a virtual environment
 which makes it also possible to run multiple versions of utm in parallel.
 
 ```bash
-pip install --index https://globaltrigger.web.cern.ch/pypi tm-python==0.14.0
+pip install --extra-index-url https://globaltrigger.web.cern.ch/pypi tm-python==0.14.0
 ```