Update help text about deleting backups

src/index.html

@@ -154,6 +154,10 @@
                                 <li>When you have opened a backup for
                                     viewing, you can easily re-open it
                                     later by selecting "Load Previous".</li>
+                                <li>Previously viewed backups can be
+                                    removed by clicking or tapping the
+                                    delete button on the right side of
+                                    the entry.</li>
                             </ul>
                         </p>