Editorial: use "active function object" more often

In #1541, a few checks on the "currently-executing constructor" were
added. The pull request review revealed that this phrase was not
sufficiently precise, so it was replaced with "active function object",
a term defined in the JavaScript specification. However, this was only
done in one of the three places the imprecise term appears. This tidies
up the other two.

source

@@ -9702,7 +9702,7 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
 
     <ol>
      <li>
-      <p>If the currently-executing constructor is not <code>HTMLElement</code>, then throw a
+      <p>If the <span>active function object</span> is not <code>HTMLElement</code>, then throw a
       <code>TypeError</code> and abort these steps.</p>
 
       <div class="example no-backref">
@@ -9712,8 +9712,8 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
        <pre>customElements.define("bad-1", class Bad1 extends HTMLParagraphElement {});</pre>
 
        <p>In this case, during the (implicit) <code data-x="">super()</code> call that occurs when
-       constructing an instance of <code data-x="">Bad1</code>, the currently-executing constructor
-       is <code>HTMLParagraphElement</code>, not <code>HTMLElement</code>.</p>
+       constructing an instance of <code data-x="">Bad1</code>, the <span>active function
+       object</span> is <code>HTMLParagraphElement</code>, not <code>HTMLElement</code>.</p>
       </div>
      </li>
     </ol>