Suggest developer uses Incognito Mode for auditing (#10854)

* Suggest developer uses Incognito Mode for auditing

As I followed this tutorial, I found that my Chrome plugins, (ad blockers, etc.) interfered with the audit. There's no way to know which plugins a user may have installed, so I think it's best to audit the Gatsby app from a clean slate.

* Explain why you'd use Incognito

docs/tutorial/part-eight/index.md

@@ -49,7 +49,7 @@ Once this starts, you can now view your site at `localhost:9000`.
 
 Now you're going to run your first Lighthouse test.
 
-1.  Open the site in Chrome (if you didn't already do so) and then open up the Chrome DevTools.
+1.  If you haven't already done so, open the site in Chrome Incognito Mode so no extensions interfere with the test. Then, open up the Chrome DevTools.
 
 2.  Click on the "Audits" tab where you'll see a screen that looks like: