Fix argument name of Enumerable::countBy. (#44775)

Author: tyabu12

src/Illuminate/Collections/Enumerable.php

@@ -1168,7 +1168,7 @@ public function count(): int;
      * @param  (callable(TValue, TKey): mixed)|string|null  $countBy
      * @return static<array-key, int>
      */
-    public function countBy($callback = null);
+    public function countBy($countBy = null);
 
     /**
      * Zip the collection together with one or more arrays.