rename data_table to datatable

[tdhock]

hi @mine-cetinkaya-rundel I mentioned in the other pr #335 that we should either delete the old or change the name of the new file to the old, and this PR does that. I saw on https://rstudio.github.io/cheatsheets/contributed-cheatsheets.html that the link is datatable.pdf so it would be good to keep the original name, so we don't have to change it elsewhere. Thanks!