AP_NavEKF: Fix incorrect units in gimbal EKF covariance prediction

This change does not change the tuning with the current time step, but means that any future changes to update rate will be less likely to cause problem with filter tuning.

libraries/AP_NavEKF/AP_SmallEKF.cpp

@@ -197,7 +197,7 @@ void SmallEKF::predictStates()
 // gyro_bias_state_noise
 void SmallEKF::predictCovariance()
 {
-    float delAngBiasVariance = sq(dtIMU*dtIMU*5E-4f);
+    float delAngBiasVariance = sq(dtIMU*5E-6f);
 
     float daxNoise = sq(dtIMU*0.0087f);
     float dayNoise = sq(dtIMU*0.0087f);