chore(docs): Update tutorial part 8

[jfrato]

Fix broken link Adding an SEO component

Description

• On page https://www.gatsbyjs.org/tutorial/part-eight/ in the link Adding an SEO Component is broken leading the user to a 404 page.

• Fixed link

Related Issues

Fixes #18511

[LekoArts]

Please commit Benjamin's suggestion. After that we can merge 👍

[LekoArts]

Thanks 👍

[gatsbot]

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