added dt to tutorial movie vis

[cooperrc]

I received an error in the tutorial

In [18]: visu = viz.Visualizer(results, pend, speed=1)                                                              
---------------------------------------------------------------------------                                         
TypeError                                 Traceback (most recent call last)                                         
Input In [18], in <cell line: 1>()                                                                                  
----> 1 visu = viz.Visualizer(results, pend, speed=1)                                                               
                                                                                                                    
TypeError: Visualizer.__init__() missing 1 required positional argument: 'dt'                                       

So I added the dt = dt argument to the Visualizer argument in the tutorial