Update tests.md

Author: sterbini

python_examples/hl_lhc_collisions_python/checks_and_doc/tests.md

@@ -82,7 +82,7 @@ tests = [
 Check using xline lines:
 ```bash
 python t003_fc_to_fort.py
-python t004_compare_xline_lines.py
+python t004_compare_xlines.py.py
 ```
 
 **Check crab cavities:**