Move heterogenous->heterogeneous to the rare dictionary

"heterogenous" is both a valid word with a distinct meaning,
and an accepted alternative spelling of "heterogeneous"
(see <https://en.wiktionary.org/wiki/heterogenous>)
so it is removed from the main substitutions dictionary.

However, "heterogenous" is much rarer in practice than "heterogeneous":
- [google ngram viewer](https://books.google.com/ngrams/graph?content=heterogenous%2Cheterogeneous)
- [4M search results](https://www.google.com/search?q=heterogenous) for "heterogenous", vs
  [111M search results](https://www.google.com/search?q=heterogeneous) for "heterogeneous"
...so it is added to the rare dictionary as word that is valid, but likely to be a typo.