Erdős Problem 501: status mismatch (repo=open, erdosproblems.com=solved)

The status of [Erdős problem 501](https://www.erdosproblems.com/501) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/501.lean)**: `open` (in `FormalConjectures/ErdosProblems/501.lean`)
- **[erdosproblems.com/501](https://www.erdosproblems.com/501)**: `solved`

Please verify and update the `@[category research ...]` annotation if appropriate.

Open FormalConjectures/ErdosProblems/501.lean and inspect the @[category research ...] annotation for Erdős problem 501. Verify the current status at erdosproblems.com/501, then update the annotation if the external status is confirmed; done means the repository status matches the verified source.

Written by the indexing model from the issue text.