spec: Add link to dependency tc39/ecma262#2942

See #36 (comment) (@bakkot).

Author: js-choi

spec.html

@@ -17,6 +17,7 @@ <h1>Introduction</h1>
   several new or revised clauses. See <a
   href=https://github.com/js-choi/proposal-array-async-from/blob/main/README.md>the proposal's
   explainer</a> for the proposal's background, motivation, and usage examples.</p>
+  <p>This proposal depends on the <a href=https://github.com/tc39/ecma262/pull/2942>pull request tc39/ecma262#2942: “support built-in async functions”</a>.</p>
 </emu-intro>
 
 <emu-clause id="sec-control-abstraction-objects">