update open styling

Co-authored-by: Yaël Dillies <yael.dillies@gmail.com>

Author: ryantuck

FormalConjectures/ErdosProblems/13.lean

@@ -22,8 +22,7 @@ import FormalConjectures.Util.ProblemImports
 *Reference:* [erdosproblems.com/13](https://www.erdosproblems.com/13)
 -/
 
-open Finset
-open Nat
+open Finset Nat
 
 namespace Erdos13