more explicit instruction to rerun npsim

src/docs/part1/simple_detector.md

@@ -248,7 +248,13 @@ In the `CartesianGridXY` case, the cell sizes are always the same.
   <id>system:5,barrel:3,layer:4,module:5,r:32:-16,phi:-16</id>
 </readout>
 ```
-2. Re-run  npsim command above.
+2. You can now re-run the npsim command above, as we changed the detector pixel layout.
+``bash
+npsim  --runType run  --enableG4GPS \
+   --macroFile gps.mac \
+   --compactFile ./gem_tracker.xml \
+   --outputFile gem_tracker_sim.root 
+``
 3. Re-run the root script:
 ```bash
 root -b -q scripts/tutorial2_cell_size.cxx+