Update tree.html

Typo "Deleted selected" -> "Delete selected"

notebook/templates/tree.html

@@ -32,7 +32,7 @@ data-terminals-available="{{terminals_available}}"
                   <button title="Duplicate selected" class="duplicate-button btn btn-default btn-xs">Duplicate</button>
                   <button title="Rename selected" class="rename-button btn btn-default btn-xs">Rename</button>
                   <button title="Shutdown selected notebook(s)" class="shutdown-button btn btn-default btn-xs btn-warning">Shutdown</button>
-                  <button title="Deleted selected" class="delete-button btn btn-default btn-xs btn-danger"><i class="fa fa-trash"></i></button>
+                  <button title="Delete selected" class="delete-button btn btn-default btn-xs btn-danger"><i class="fa fa-trash"></i></button>
               </div>
             </div>
             <div class="col-sm-4 no-padding tree-buttons">