[w3c/permissions] Auto-fetch the previous version link. (#219)

This will allow echidna to keep the spec updated.

THIS IS CURRENTLY BROKEN: it links to `permissions-policy-1` instead of `permissions`.


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<a href="https://pr-preview.s3.amazonaws.com/jyasskin/permissions/pull/219.html" title="Last updated on Jul 22, 2020, 4:36 PM UTC (520ee29)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/permissions/219/96646df...jyasskin:520ee29.html" title="Last updated on Jul 22, 2020, 4:36 PM UTC (520ee29)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/permissions/pull/219

-- Commit Summary --

  * Auto-fetch the previous version link.

-- File Changes --

    M index.bs (3)

-- Patch Links --

https://github.com/w3c/permissions/pull/219.patch
https://github.com/w3c/permissions/pull/219.diff

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