Ch5: Fix python code in Priceline showdown example

Name the "sp3" plot so it may be referenced in the label.
Fix indexing assignment error in showdown_loss() by creating a "close_mask"
intermediate boolean indexing variable.
Fix risk assignment -- values over by less than $250 were not being assigned the risk value. Ensure this happens by assigning loss[ix] = risk last.
Outputs and plots are not changed.
Update README.md