Updated B scan example to match that in docs.

README.rst

@@ -208,11 +208,11 @@ For example, to check the geometry of a model:
 
     python -m gprMax user_models/heterogeneous_soil.in --geometry-only
 
-For example, to run a B-scan with 54 traces:
+For example, to run a B-scan with 60 traces:
 
 .. code-block:: none
 
-    python -m gprMax user_models/GSSI_1500_cylinder_Bscan.in -n 54
+    python -m gprMax user_models/GSSI_1500_cylinder_Bscan.in -n 60