Erdős Problem 397: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 397](https://www.erdosproblems.com/397) appears to have changed.

- **This repo**: `solved` (in `FormalConjectures/ErdosProblems/397.lean`)
- **erdosproblems.com**: `formally solved`

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