fix some broken marketplace-related links

[paulfitz]

Ran across a broken link. Fixed these warnings during build:

WARNING:mkdocs.structure.pages:Doc file 'install/aws-marketplace-legacy.md' contains a relative link 'aws-marketplace.md', but the target 'install/aws-marketplace.md' is not found among documentation files.
WARNING:mkdocs.structure.pages:Doc file 'install/aws-marketplace-legacy.md' contains a relative link 'aws-marketplace.md#authentication-setup', but the target 'install/aws-marketplace.md' is not found among documentation files.
WARNING:mkdocs.structure.pages:Doc file 'install/aws-marketplace-transition.md' contains a relative link 'aws-marketplace.md', but the target 'install/aws-marketplace.md' is not found among documentation files.

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[jordigh]

Thanks. I think they were handled by redirects anyway, but it's good to silence the warnings.

[paulfitz]

Thanks. I think they were handled by redirects anyway, but it's good to silence the warnings.
Turns out the page with the broken links is still reachable if you're googling and the redirects didn't help them.