Fix label IDs on join form

frontend/index.html

@@ -16,7 +16,7 @@
       <form id="join-session-form">
         <h2>Join a session</h2>
 
-        <label for="nickname">Nickname:</label>
+        <label for="join-session-nickname">Nickname:</label>
         <input
           type="text"
           id="join-session-nickname"
@@ -24,7 +24,7 @@
           required
         />
 
-        <label for="session-id">Session ID:</label>
+        <label for="join-session-id">Session ID:</label>
         <input
           type="text"
           id="join-session-id"