textstat_readability() returns NA for Bormuth and DRP

[kbenoit]

> textstat_readability(data_corpus_inaugural[2], measure = "Bormuth")
         document Bormuth.MC
1 1793-Washington         NA
> textstat_readability(data_corpus_inaugural[2], measure = "DRP")
         document DRP
1 1793-Washington  NA

[jiongweilua]

@kbenoit I can check this out if you are busy with other stuff for the package

[kbenoit]

That'd be awesome!