Airspeed Library - fix param names.

The Group definition give the start of the param name.
This way we can maintain multiple instances if required of the same library as options.

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -16,25 +16,25 @@
 // table of user settable parameters
 const AP_Param::GroupInfo AP_Airspeed::var_info[] PROGMEM = {
 
-    // @Param: ARSPD_ENABLE
+    // @Param: ENABLE
     // @DisplayName: Airspeed enable
     // @Description: enable airspeed sensor
     // @Values: 0:Disable,1:Enable
     AP_GROUPINFO("ENABLE",    0, AP_Airspeed, _enable),
 
-    // @Param: ARSPD_USE
+    // @Param: USE
     // @DisplayName: Airspeed use
     // @Description: use airspeed for flight control
     // @Values: 0:Use,1:Don't Use
     AP_GROUPINFO("USE",    1, AP_Airspeed, _use),
 
-    // @Param: ARSPD_OFFSET
+    // @Param: OFFSET
     // @DisplayName: Airspeed offset
     // @Description: Airspeed calibration offset
     // @Increment: 0.1
     AP_GROUPINFO("OFFSET", 2, AP_Airspeed, _offset),
 
-    // @Param: ARSPD_RATIO
+    // @Param: RATIO
     // @DisplayName: Airspeed ratio
     // @Description: Airspeed calibration ratio
     // @Increment: 0.1