[editing-explainer] Fix URL of ContentEditable spec. (#45)

The URL for the ContentEditable spec is missing its scheme part, thus attempts to access it return a 404 error.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/editing-explainer/pull/45

-- Commit Summary --

  * Fix URL of ContentEditable spec.

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/w3c/editing-explainer/pull/45.patch
https://github.com/w3c/editing-explainer/pull/45.diff

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https://github.com/w3c/editing-explainer/pull/45