change name of standard out option for clarity

This is a quick change to one of the MCSim input options regarding the standard output. The current name save_standard_out is a bit of a misnomer since this option doesn't actually save the OpenMC output to a file but instead prints it to the screen while running. Also, it was the only remaining option in the output{} block so it simplified in that respect as well. Option has been changed to simply print_output. Functionally this is still doing the same thing as before despite the slightly incorrect old description.