use correct typehint for callable

Author: willemwollebrants

src/Valitron/Validator.php

@@ -1229,7 +1229,7 @@ public function hasValidator($name)
     /**
      * Convenience method to add a single validation rule
      *
-     * @param string|callback $rule
+     * @param string|callable $rule
      * @param array|string $fields
      * @return Validator
      * @throws \InvalidArgumentException