Erdős Problem 2: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 2](https://www.erdosproblems.com/2) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/2.lean)**: `solved` (in `FormalConjectures/ErdosProblems/2.lean`)
- **[erdosproblems.com/2](https://www.erdosproblems.com/2)**: `formally solved`

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

Start with FormalConjectures/ErdosProblems/2.lean and inspect its @[category research ...] annotation. Verify the current status against erdosproblems.com/2, then update the annotation if the external status confirms a mismatch. Done means the repository metadata agrees with the verified status.

Written by the indexing model from the issue text.