Erdős Problem 481: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 481](https://www.erdosproblems.com/481) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/481.lean)**: `solved` (in `FormalConjectures/ErdosProblems/481.lean`)
- **[erdosproblems.com/481](https://www.erdosproblems.com/481)**: `formally solved`

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

Start with FormalConjectures/ErdosProblems/481.lean and inspect its @[category research ...] annotation. Compare the repository status with the linked erdosproblems.com/481 entry, then update the annotation if the external status is confirmed and verify the resulting Lean file.

Written by the indexing model from the issue text.