[webcomponents] [Custom] `document.register` is a typo (#345)

In [section 7.1](https://w3c.github.io/webcomponents/spec/custom/#dfn-document-registerElement):

> ElementRegistrationOptions is an abstraction that enables using function objects and ES6 classes as the second argument of document.register method.

It should say `document.registerElement`.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/345