Added correct GPS link

src/docs/part1/simple_detector.md

@@ -141,7 +141,7 @@ This will run geant4 using the General Particle Source (GPS) tool.
 The output will be a root file containing the generated events and GEM tracker hits.
 Look at [gps.mac](https://eicweb.phy.anl.gov/EIC/tutorials/ip6_tutorial_1/-/blob/master/gps.mac).
 
-Refer to the [GPS Documentation](http://www.fe.infn.it/u/paterno/Geant4_tutorial/slides_further/GPS/GPS.pdf) for more setup possibilities.
+Refer to the [GPS Documentation](https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/GettingStarted/generalParticleSource.html?highlight=gps#macro-commands) for more setup possibilities.
 
 ## Using detector description in analysis