Update error message in `request.js`

Replace

`Global "fetch" not found. Please provide `options.request.fetch` to octokit or upgrade to node@18 or newer.`

with

`fetch is not set. Please pass a fetch implementation as new Octokit({ request: { fetch }}). Learn more at https://github.com/octokit/octokit.js/#fetch-missing`