Update puppeteer.mdx

[kathayl]

set custom user agent on puppeteer

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/browser-rendering/	@mchenco, @cloudflare/pcx-technical-writing, @celso, @danielgek, @kathayl, @ToriLindsay

[github-actions]

Preview URL: https://9e4791b7.preview.developers.cloudflare.com
Preview Branch URL: https://kathayl-patch-15.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/browser-rendering/platform/playwright/	https://kathayl-patch-15.preview.developers.cloudflare.com/browser-rendering/platform/playwright/
https://developers.cloudflare.com/browser-rendering/platform/puppeteer/	https://kathayl-patch-15.preview.developers.cloudflare.com/browser-rendering/platform/puppeteer/