fix RF param names

Author: Hwurzburg

dev/source/docs/adding_simulated_devices.rst

@@ -90,8 +90,8 @@ following commands:
     param set RNGFND1_TYPE 1
     param set RNGFND1_SCALING 10
     param set RNGFND1_PIN 0
-    param set RNGFND1_MAX_CM 5000
-    param set RNGFND1_MIN_CM 0
+    param set RNGFND1_MAX 50
+    param set RNGFND1_MIN 0
 
     # Enable rangefinder for landing (Plane only!)
     param set RNGFND_LANDING 1