Return proper number of velocity elements

Before, we sliced the velocity vector based on how many elements were in
the position vector, when this should actually use the number of
elements in the velocity vector.

environment/mujoco/internal/mujocoenv/MujocoEnv.go

@@ -271,7 +271,7 @@ func (m *MujocoEnv) QPos() []float64 {
 
 // QVel returns the current velocity of the model
 func (m *MujocoEnv) QVel() []float64 {
-	return F64SliceC2Go(m.Data.qvel, m.Nq)
+	return F64SliceC2Go(m.Data.qvel, m.Nv)
 }
 
 // SetState sets the underlying position and velocity for the model