[webcomponents] Fix typos (#379)

`preceeding` → `preceding`
`instantation` → `instantiation`
Remove extra `element`
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/379

-- Commit Summary --

  * Fix typos

-- File Changes --

    M spec/custom/index.html (6)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/379.patch
https://github.com/w3c/webcomponents/pull/379.diff

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https://github.com/w3c/webcomponents/pull/379