add example to placeholder text of group DM invitation form

following.html

@@ -421,7 +421,7 @@
             <button class="invite">Invite peers</button>
             <button class="leave">Leave group</button>
             <div class="invite-form">
-                <textarea></textarea><button disabled="disabled">Invite</button>
+                <textarea placeholder="@peer1 @peer2 …"></textarea><button disabled="disabled">Invite</button>
             </div>
         </div>
     </div>
@@ -441,7 +441,7 @@
                 <button class="show-secret-key" disabled="disabled">Secret Key</button>
                 <button class="leave" disabled="disabled">Leave group</button>
                 <div class="invite-form">
-                    <textarea></textarea><button disabled="disabled">Invite</button>
+                    <textarea placeholder="@peer1 @peer2 …"></textarea><button disabled="disabled">Invite</button>
                 </div>
                 <div class="secret-key"></div>
             </div>

home.html

@@ -513,7 +513,7 @@
             <button class="invite">Invite peers</button>
             <button class="leave">Leave group</button>
             <div class="invite-form">
-                <textarea></textarea><button disabled="disabled">Invite</button>
+                <textarea placeholder="@peer1 @peer2 …"></textarea><button disabled="disabled">Invite</button>
             </div>
         </div>
     </div>
@@ -533,7 +533,7 @@
                 <button class="show-secret-key" disabled="disabled">Secret Key</button>
                 <button class="leave" disabled="disabled">Leave group</button>
                 <div class="invite-form">
-                    <textarea></textarea><button disabled="disabled">Invite</button>
+                    <textarea placeholder="@peer1 @peer2 …"></textarea><button disabled="disabled">Invite</button>
                 </div>
                 <div class="secret-key"></div>
             </div>