[webcomponents] Update Slots-Proposal.md (#285)

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/285

-- Commit Summary --

  * Update Slots-Proposal.md

-- File Changes --

    M proposals/Slots-Proposal.md (2)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/285.patch
https://github.com/w3c/webcomponents/pull/285.diff

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https://github.com/w3c/webcomponents/pull/285