`ten_lang_percent` erroneous holdover from R

In chapter 1, we reference `ten_lang_percent` but never compute it. I believe this was just a variable name change from the R conversion that we didn't propagate to the text (and didn't catch because it caused no bugs in the code), but worth doing a small double check of the code in the py version. 

(thanks to Hossein Saiedian from the University of Kansas for noting that one!)