Erdős Problem 868: status mismatch (repo=open, erdosproblems.com=solved)

The status of [Erdős problem 868]appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/868.lean)**: `open` (in `FormalConjectures/ErdosProblems/868.lean`)
- **[erdosproblems.com/868](https://www.erdosproblems.com/868)**: `solved`

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