Update doc link to point at ECMAScript 2018.
Update doc link to point at ECMAScript 2018.

After the work done for https://github.com/dart-lang/sdk/issues/34935 and other
related issues, we should handle all of the new RegExp featuers in ECMAScript
2018. However, we still link to the ECMAScript 2011 specification. This
corrects the link.

Closes https://github.com/dart-lang/sdk/issues/23047.

Change-Id: If7dd8411d2f425caf1f03a49807121a0d61d96af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108272
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Teagan Strickland <cce218d042444252b7ac446d60910fca7732559c@google.com>

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