[Workers] Fix migrate-from-pages redirect source path in _redirect

[NEKOYASAN]

Summary

Fixed an incorrect redirect setting for the Migrate from Pages page whose URL was changed in #22064.

Screenshots (optional)

Documentation checklist

• 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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[GregBrimble]

Thank you, but this was intentional — before the other migration guides, this lived at the top of the static assets section, so I'm just trying to capture references of when it lived there.

[NEKOYASAN]

I thought it was a mistake because I was using this URL when migrate-from-pages was under migrations and not directly under static-assets.
If the current one is also correct, then it seems to me that a redirect should be set up, since the URL that is used as the redirect source for this PR is also widely known.

[GregBrimble]

It was moved under migrations four days ago, and then moved to migration-guides three days ago. I don't think many external places picked up the migrations location during that 24hrs, but if there's anywhere that does have a broken link because of this, we absolutely can include a redirect!