docs: Update broken anchor link

[unscripted]

What?

This PR updates the link to point to the correct anchor on the reference page.

Why?

When users clicked the existing link, they were directed to the top of the page. This was caused by the URL not matching the updated destination anchor.

How?

This PR updates the URL to match the destination anchor.

Testing Instructions

1. View the packages/create-block/README.md file.
2. Click the "be installed manually" link.
3. The expected result should anchor you to the Manual Plugin Installation section of the Manage Plugins post.

Testing Instructions for Keyboard

1. Press tab until you reach the "be installed manually" link.
2. Press enter to follow the link.

[github-actions]

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[alexstine]

@unscripted Thanks for the PR. This looks good.