update examples to address needed processing

[thescientist13]

related to #7

Given the current examples in the README, e.g.

const rehypePrism = require('@mapbox/rehype-prism');
const remarkParse = require('remark-parse');
const remarkRehype = require('remark-rehype');

const processedMarkdown = await unified()
  .use(remarkParse)
  .use(remarkeRhype)
  .use(rehypePrism)
  .process(/* some markdown */);

I was getting an error

Error: Cannot `process` without `Compiler`

Updated examples to include adding something like

.use(rehypeStringify)

at the end of the chain before process is called