Add missing variable definition

webrtc.html

@@ -848,6 +848,7 @@ <h4>Constructor</h4>
             internal slot, initialized to the <a data-cite="html">current settings object</a>'s <a data-cite="html#concept-settings-object-origin">
             origin</a>.</p>
           </li>
+          <li>Let <var>configuration</var> be the method's first argument.</li>
           <li data-tests="RTCCertificate.html">
             <p id="certificate-validation">If the {{RTCConfiguration/certificates}} value in
             <var>configuration</var> is non-empty, run the following steps for