$ref objects are legal in all path-item objects

See #2657 - this was added to the specification but not the schema.

schemas/v3.1/schema.yaml

@@ -192,7 +192,7 @@ $defs:
     type: object
     patternProperties:
       '^/':
-        $ref: '#/$defs/path-item'
+        $ref: '#/$defs/path-item-or-reference'
     $ref: '#/$defs/specification-extensions'
     unevaluatedProperties: false