Update README because the code is no longer in a single file

[arks-public]

The README has a section for "Single File Installing", even though the code is no longer in a single file.

This README should indicate that version 5.1.0 is the last version with FreeSimpleGUI.py -> https://github.com/spyoungtech/FreeSimpleGUI/blob/d8c06c2bbe6d51425d1b0d79557b76f99b933708/FreeSimpleGUI/FreeSimpleGUI.py

Otherwise, this section could be removed altogether