Editorial: Use "well-known symbol" term consistently (tc39#3236)

Fixes tc39#3231.

spec.html

@@ -30565,7 +30565,7 @@ <h1>Properties of the Symbol Constructor</h1>
 
       <emu-clause id="sec-symbol.asynciterator">
         <h1>Symbol.asyncIterator</h1>
-        <p>The initial value of `Symbol.asyncIterator` is the well known symbol @@asyncIterator (<emu-xref href="#table-well-known-symbols"></emu-xref>).</p>
+        <p>The initial value of `Symbol.asyncIterator` is the well-known symbol @@asyncIterator (<emu-xref href="#table-well-known-symbols"></emu-xref>).</p>
         <p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>
       </emu-clause>