fix typo in code

client/js/client.js

@@ -90,7 +90,7 @@ function showGroups(payload) {
     let list = groupElm.querySelector("ul");
     for ( let member in group.members ) {
 	let memberElm = document.createElement("li");
-	memberElm.classList.append( "list-group-item" );
+	memberElm.classList.add( "list-group-item" );
 	memberElm.innerText = group.members[member];
 	list.appendChild(memberElm);
     }