(minor) Double backtick typo to triple backtick

src/docs/part1/simple_detector.md

@@ -249,12 +249,12 @@ In the `CartesianGridXY` case, the cell sizes are always the same.
 </readout>
 ```
 2. You can now re-run the npsim command above, as we changed the detector pixel layout.
-``bash
+```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+