The .txt files are gone, you will need to generate the .rtf files from the .html files - consequently edit the .html files, not the generated .rtf files. 
The conversions should have been taken care of in the Makefile.

The conversion is done by the 'textutil' command line utility, included in OS X.

example:
    textutil -convert rtf introduction.html

which will output a file:
   introduction.rtf

All .rtf files are git ignored in this folder.