allow schema of MediaTypeObject to be a ReferenceObject as well

[kelvin-thinkbean]

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[kelvin-thinkbean]

Hello, this PR fixes the following:

import { OpenAPIV3_1 } from 'openapi-types'

const requestBody: OpenAPIV3_1.RequestBodyObject = {
  required: true,
  description: 'Login information',
  content: {
    'application/json': {
      schema: {
        $ref: '#/components/schemas/LoginInfoSchema',
      },
    },
  },
}

Error: