AC_Sprayer: use vector.xy().length() instead of norm(x,y)

libraries/AC_Sprayer/AC_Sprayer.cpp

@@ -132,7 +132,8 @@ void AC_Sprayer::update()
         // velocity will already be zero but this avoids a coverity warning
         velocity.zero();
     }
-    float ground_speed = norm(velocity.x * 100.0f, velocity.y * 100.0f);
+
+    float ground_speed = velocity.xy().length() * 100.0;
 
     // get the current time
     const uint32_t now = AP_HAL::millis();