Erdős Problem 13: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 13](https://www.erdosproblems.com/13) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/13.lean)**: `solved` (in `FormalConjectures/ErdosProblems/13.lean`)
- **[erdosproblems.com/13](https://www.erdosproblems.com/13)**: `formally solved`

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

Open FormalConjectures/ErdosProblems/13.lean and inspect the @[category research ...] annotation for problem 13. Compare it with the status shown at erdosproblems.com/13, then update the annotation if the external status is confirmed. Done means the repository status matches the verified problem status.

Written by the indexing model from the issue text.