changed fa -> fas in the twotothree mapping file

This is a bold move.  I don't want to change the source file, although my research seems to indicate that fa is a macro language code for Persian (Farsi) and the corresponding three letter macrolanguage code is fas.
See: http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
See: http://en.wikipedia.org/wiki/ISO_639_macrolanguage#fas