docs: Fix simple typo, underlaying -> underlying
docs: Fix simple typo, underlaying -> underlying

There is a small typo in docs/reference/image.rst.

Should read `underlying` rather than `underlaying`.

jazzband/sorl-thumbnail,mariocesar/sorl-thumbnail,jazzband/sorl-thumbnail,mariocesar/sorl-thumbnail,jazzband/sorl-thumbnail,mariocesar/sorl-thumbnail