Update spec language to refer to child navigables instead of nested browing contexts?

[markafoltz]

The term nested browsing context has apparently been removed in the WhatWG HTML spec and replaced with the concept of a child navigable [1, 2]. I believe we can do a 1-1 replacement of the terms without changing the meaning of anything, but need to check.

Right now, ReSpec is complaining that it can't find an x-ref for nested browsing context as that term no longer exists in WhatWG HTML.

[1] whatwg/html#6315
[2] https://html.spec.whatwg.org/multipage/document-sequences.html#child-navigable