Fix type emitted in declaration file

This is required for TypeScript to avoid referencing the wrong type (see microsoft/TypeScript#48242 (comment)).

lib/react-markdown.js

@@ -12,7 +12,7 @@
  * @typedef PluginOptions
  * @property {PluggableList} [remarkPlugins=[]]
  * @property {PluggableList} [rehypePlugins=[]]
- * @property {import('remark-rehype').Options} [remarkRehypeOptions={}]
+ * @property {import('remark-rehype').Options | undefined} [remarkRehypeOptions={}]
  *
  * @typedef LayoutOptions
  * @property {string} [className]