docs(github-pages.md): Fix deployment example

[Aervyon]

The Problem

The previous version of github-pages.md had a deployment example that would generate a Nuxt server, instead of a static site.

How does this fix the issue?

To fix this I simply explicitly defined the environment variable that the Nitro Github Pages Deployment template showcased.

This fixes the issue of the workflow file generating a Nuxt server instead of a static site.

[nuxt-studio[bot]]

✅ Live Preview ready!

Name	Edit	Preview	Latest Commit
nuxt.com	Edit on Studio ↗︎	View Live Preview	0cc4b7e5b34ac0ff69e16825ed8baecff4dd1c0c

[atinux]

Should be fixed by using npx nuxt build --preset github_pages for npm we need to add another --: npm run build -- --preset github_pages