Broken link in the documentation

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If you have a bug for AMP please fill in the following template. Delete everything except the headers (including this text).

## What's the issue?

Briefly describe the bug.

broken link for story poster documentation.

## How do we reproduce the issue?
https://amp.dev/documentation/components/amp-story/#attributes
go to poster-portrait-src (required) and try to click on Story Poster blue link and you'll get a 404. it's a link to https://github.com/ampproject/amphtml/blob/master/extensions/amp-story/%3C#poster-guidelines-(for-poster-portrait-src,-poster-landscape-src,-and-poster-square-src)%3E

Please provide a public URL and ideally a reduced test case (e.g. on jsbin.com) that exhibits only your issue and nothing else. Provide step-by-step instructions for reproducing the issue:

1. Step 1 to reproduce
2. Step 2 to reproduce
3. …

## What browsers are affected?

All browsers? Some specific browser? What device type?

## Which AMP version is affected?

Is this a new issue? Or was it always broken? Paste the version of AMP where you saw this issue. (You can find the version printed in your browser's console.)

Start at the AMP story documentation URL and inspect the poster-portrait-src (required) section, specifically its Story Poster link. Replace the broken destination with a valid documentation link and verify that it no longer returns a 404.

Written by the indexing model from the issue text.