Update Marketplace OpenAPI doc url

What does this MR do and why?

For issue https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5314+

With MR MR https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6522+ deployed, the Marketplace OpenAPI doc is now publicly available at: https://customers.staging.gitlab.com/openapi_docs/marketplace

This MR update this URL on the Marketplace partner integration overview page.

How to set up and validate locally

1. Follow the GDK document to view gitlab docs locally
2. Go to gitlab-docs project, check out this MR's branch qzhao-update-marketplace-openapi-doc-url
3. gdk start
4. Visit http://127.0.0.1:3005/ee/integration/partner_marketplace.html#marketplace-api-specification . Click the Marketplace interactive API documentation, it will open https://customers.staging.gitlab.com/openapi_docs/marketplace.

I am including the page snapshot here, so you do not have to bother the gitlab-docs local setup.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.