[w3c/manifest] Bad cross references (#884)

Couple of bad cross references... we are probably trying to cite things that don't exist anymore or that are not explicitly exported by HTML.
 
```
https://html.spec.whatwg.org/multipage/#unordered-set-of-unique-space-separated-tokens
https://html.spec.whatwg.org/multipage/#linkTypes
https://html.spec.whatwg.org/multipage/#delay-the-load-event
https://html.spec.whatwg.org/multipage/#valid-non-negative-integer
https://html.spec.whatwg.org/multipage/#concept-link-type-sniffing
https://html.spec.whatwg.org/multipage/#the-element's-base-url
```


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