fix(gatsby-plugin-mdx): remark/hypePlugins options schema

[mxstbr]

The documented form to configure gatsby-plugin-mdx with remark/hypePlugins is to require them:

{
  resolve: `gatsby-plugin-mdx`,
  options: {
    remarkPlugins: [require(`remark-slug`)]
  }
}

The current pluginOptionsSchema for gatsby-plugin-mdx throws an error with this (valid!) configuration, stating that "remarkPlugins[0] should be object or array", which is inaccurate.

This fixes it by adding the correct type (function) to the possibilities for this array. Verified it works as expected by running the fix against the theme-ui docs website, which fails before this fix but works as expected afterwards.

[mxstbr]

This is a typo, as the option is actually called defaultLayouts.