MRF tutorials: Change rotor velocity boundary condition to fixedValue

Using the noSlip boundary condition for rotating wall in an MRF region
interferes with post-processing by resetting the wall velocity to 0 rather than
preserving the value set by the MRF zone.

tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D/0/U

@@ -22,7 +22,8 @@ boundaryField
 {
     rotor
     {
-        type            noSlip;
+        type            fixedValue;
+        value           uniform (0 0 0);
     }
 
     stator