re-do the binder link

Author: peremato

README.md

@@ -4,7 +4,7 @@ Materials for the Geant4.jl Tutorial.
 Read the rendered Jupyter book: https://peremato.github.io/Geant4.jl-tutorial/index.html
 
 Follow the tutorial with Binder:
-[![Binder](https://binderhub.ssl-hep.org/badge_logo.svg)](https://binderhub.ssl-hep.org/v2/gh/peremato/Geant4.jl-tutorial/HEAD?labpath=tutorial%2Fdocs%2Fgeant4-jl-intro.ipynb)
+[![Binder](https://binderhub.ssl-hep.org/badge_logo.svg)](https://binderhub.ssl-hep.org/v2/gh/peremato/Geant4.jl-tutorial/HEAD?labpath=tutorial%2Fdocs%2F01-introduction.ipynb)
 
 - - -
 When not taken from other sources (with its own Copyright and License), this material is: