Erdős Problem 394: status mismatch (repo=formally solved, erdosproblems.com=open)

The status of [Erdős problem 394](https://www.erdosproblems.com/394) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/394.lean)**: `formally solved` (in `FormalConjectures/ErdosProblems/394.lean`)
- **[erdosproblems.com/394](https://www.erdosproblems.com/394)**: `open`

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

Open FormalConjectures/ErdosProblems/394.lean and inspect its @[category research ...] annotation. Verify the current status against erdosproblems.com/394, then update the annotation if the problem is no longer open; done means the repository metadata agrees with the verified external status.

Written by the indexing model from the issue text.