Fix link to constraint validation API

[bryik]

I was reading your lovely docs and noticed this link has died. It seems W3C moved their documentation; the new address is here.

[mdo]

Unsure about the github.io address as w3.org still works for me as the primary domain. What about https://dev.w3.org/html5/spec-preview/constraints.html?

[bryik]

I found a few links to W3C constraint validation API docs and wasn't sure which to pick. WHATWG has a version as well--confusing!

@mdo if that is the link you want, I'll update the PR

[wolfy1339]

The github.io seems to be more up to date then the other addresses
I did find the new link for the page: https://www.w3.org/TR/html5/sec-forms.html#the-constraint-validation-api

[XhmikosR]

@mdo: I say you just edit the file on this PR and switch to the link @wolfy1339 suggested above.

PS. I will run the broken link checker branch which should show if we have other such errors.

[mdo]

Closing for #25244 to do just that.