126be04 Change old mention of %__python to %__python3

Authored and Committed by tibbs 4 years ago
    Change old mention of %__python to %__python3
    
    %__python was being used as an example of a valid macro for a system
    executable, but that kind of conflicts with our efforts to get rid of
    the unversioned python macro.  This is a quick fix so that the section
    isn't totally wrong.