$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