Editorial: Fix the return-type of CompilePattern (tc39#2698)

PR tc39#1713 changed the signature of the Abstract Closure
created+returned by CompilePattern,
but didn't make the corresponding change
to the return-type in the section heading.

spec.html

@@ -35082,7 +35082,7 @@ <h1>Notation</h1>
       </emu-clause>
 
       <emu-clause id="sec-compilepattern" type="sdo" oldids="sec-pattern">
-        <h1>Runtime Semantics: CompilePattern ( ): an Abstract Closure that takes a String and a non-negative integer and returns a MatchResult</h1>
+        <h1>Runtime Semantics: CompilePattern ( ): an Abstract Closure that takes a List of characters and a non-negative integer and returns a MatchResult</h1>
         <dl class="header">
         </dl>
         <emu-grammar>Pattern :: Disjunction</emu-grammar>