Erdős Problem 851: status mismatch (repo=open, erdosproblems.com=solved)

[github-actions]

The status of Erdős problem 851 appears to have changed.

• This repo: open (in FormalConjectures/ErdosProblems/851.lean)
• erdosproblems.com: solved

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