[dom] Possible typo: "as a node tree" to "as node trees" (#113)

In [&sect; 4.1](https://dom.spec.whatwg.org/#introduction-to-the-dom): "These documents are presented as <strong><strike>a</strike></strong> node tree<strong>s</strong>".

That may not be the intended meaning, however. In which case, please ignore.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/113