Update running.scrbl

Added a method for using Pyret in a standard terminal window, which is not solved by the .bashrc method on OSX 10.8 (and probably as low as 10.6, but I have no way to test this)

Instructions originally from http://architectryan.com/2012/10/02/add-to-the-path-on-mac-os-x-mountain-lion/#.Ul3v55SxNU-
and adapted for the specific situation