ENH: streamLines: updated syntax for functionObject

tutorials/incompressible/pisoFoam/les/motorBike/motorBike/system/streamLines

@@ -17,7 +17,7 @@ streamLines
     setFormat       vtk; //gnuplot; //xmgr; //raw; //jplot; //csv; //ensight;
 
     // Velocity field to use for tracking.
-    U U;
+    UName U;
 
     // Tracked forwards (+U) or backwards (-U)
     trackForward    true;