[heycam/webidl] update Ecmarkup link (#973)

Ecmarkup link is currently 404. Update to the new page for it, at https://tc39.es/ecmarkup/ .
You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/973

-- Commit Summary --

  * update Ecmarkup link

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/heycam/webidl/pull/973.patch
https://github.com/heycam/webidl/pull/973.diff

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