Move `dateset` to dictionary_code.txt

Although it's hardly a word, it appears to have been added
to the aspell dictionary:
https://github.com/codespell-project/codespell/issues/2897

Not sure what the rationale for adding `dateset` to the aspell
dictionary is, but search engines show the context is usually
programming and source code, so `dictionary_code.txt` is probably
the best choice.