Fix miscounted number of custom element callbacks

ab73443 introduced new custom element
callbacks, but continued to referring to there being four callbacks.
Let's stop stating how many callbacks there are.

Fixes #4822.

source

@@ -67110,7 +67110,7 @@ document.body.append(parent);
 
    <dt>A collection of <dfn
    data-x="concept-custom-element-definition-lifecycle-callbacks">lifecycle callbacks</dfn></dt>
-   <dd>A map, whose four keys are the strings "<code data-x="">connectedCallback</code>",
+   <dd>A map, whose keys are the strings "<code data-x="">connectedCallback</code>",
    "<code data-x="">disconnectedCallback</code>", "<code data-x="">adoptedCallback</code>",
    "<code data-x="">attributeChangedCallback</code>",
    "<code data-x="">formAssociatedCallback</code>",
@@ -67332,13 +67332,13 @@ dictionary <dfn>ElementDefinitionOptions</dfn> {
      <li><p>If <span data-x="js-Type">Type</span>(<var>prototype</var>) is not Object, then throw a
      <code>TypeError</code> exception.</p></li>
 
-     <li><p>Let <var>lifecycleCallbacks</var> be a map with the four keys "<code
+     <li><p>Let <var>lifecycleCallbacks</var> be a map with the keys "<code
      data-x="">connectedCallback</code>", "<code data-x="">disconnectedCallback</code>", "<code
      data-x="">adoptedCallback</code>", and "<code data-x="">attributeChangedCallback</code>", each
      of which belongs to an entry whose value is null.</p>
 
      <li>
-      <p>For each of the four keys <var>callbackName</var> in <var>lifecycleCallbacks</var>, in the
+      <p>For each of the keys <var>callbackName</var> in <var>lifecycleCallbacks</var>, in the
       order listed in the previous step:</p>
 
       <ol>