[webcomponents] Fix various typos (#456)

Some quick typo fixes:

* Custom **c**lements -> Custom elements
* CustomElementRegistry -> CustomElement**s**Registry (both are used interchangably, but the latter much more commonly, and it matches its `window.customElements` property name.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/456

-- Commit Summary --

  * Fix small typos

-- File Changes --

    M spec/custom/index.html (10)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/456.patch
https://github.com/w3c/webcomponents/pull/456.diff

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