Incorrect documentation for `filter` on `uploadHandler`

[dcramer]

What version of Remix are you using?

1.6.5

Steps to Reproduce

Previously filter had mimetype in the params, and the documentation still reflects this. However the new parameter is named contentType.

https://remix.run/docs/en/v1/api/remix#unstable_createfileuploadhandler-node

[kiliman]

@dcramer feel free to submit a documentation PR. Make sure you're on the main branch.