[rotorcraft] CAUTION! ref_quat scaling fix

Attitude reference quaternion:
Fix scale of angle in 2nd order model, since error quaternion contains the half-angles we get 2*omega^2*err

This means that you should halve your REF_OMEGA_[PQR] parameters to get nearly the same as before.
[conf] rotorcrafts: update REF_OMEGA_x in examples

to half the value than before