[webcomponents] Fix typo and add missing selectors (#295)

There were a couple of typos in here (specifaction, selectores, :shadow), and I couldn't quite understand the second sentence in the note, so I took a stab at rephrasing. Also, I think :host and :host() should be mentioned here.

I realize this section will be changing, so feel free to disregard this PR if it's not useful.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/295

-- Commit Summary --

  * Fix typo and add missing selectors

-- File Changes --

    M spec/shadow/index.html (5)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/295.patch
https://github.com/w3c/webcomponents/pull/295.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/295