[w3c/ServiceWorker] Link to v1 in README.md (#1043)

CONTRIBUTING.md had the right link, but this didn't.

Followup to commit 3d10320a49ca9cef533d3ca3b2b5ca7677076b36.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1043

-- Commit Summary --

  * Link to v1 in README.md

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1043.patch
https://github.com/w3c/ServiceWorker/pull/1043.diff

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