Update client_js.html

fix typo

Author: conshus

app-to-phone-js/client_js.html

@@ -68,7 +68,7 @@ <h1>Call Phone from App</h1>
     hangUpButton.addEventListener("click", () => {
       client.hangup(callId)
         .then(() => {
-          hangupButton.style.display = "none";
+          hangUpButton.style.display = "none";
           callButton.style.display = "inline";
         })
         .catch(error => {