fix: break words on hyphens for the an_a lint

[alex-ozdemir]

The sentence "This is not an ML-based system." is grammatical, because the an/a vowel rule applies to "ML" (an initialism).

Another approach would be to add a new document iterator (similar to iter_words) that splits on hyphens too, and use that iterator.

[elijah-potter]

Looks great! Thanks for fixing it up.