Erdős Problem 516: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 516](https://www.erdosproblems.com/516) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/516.lean)**: `solved` (in `FormalConjectures/ErdosProblems/516.lean`)
- **[erdosproblems.com/516](https://www.erdosproblems.com/516)**: `formally solved`

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

Compare the status shown on erdosproblems.com/516 with the @[category research ...] annotation in FormalConjectures/ErdosProblems/516.lean. Verify whether the repository’s status is accurate, then update the annotation if needed; the task is done when the two status descriptions agree.

Written by the indexing model from the issue text.