bugfix:Binder example Timoshenko

This commit fixes a bug in Timoshenko beam Binder example. Dynamic plotting was not working
previously due to the misuse of `do_step`. This commit fixes this bug.

examples/Binder/1_Timoshenko_Beam.ipynb

@@ -393,12 +393,12 @@
     "    from elastica.timestepper import extend_stepper_interface\n",
     "    from elastica.timestepper.symplectic_steppers import PositionVerlet\n",
     "    timestepper = PositionVerlet()\n",
-    "    extend_stepper_interface(timestepper, simulator)\n",
+    "    do_step, stages_and_updates = extend_stepper_interface(timestepper, simulator)\n",
     "\n",
     "    n_steps = int((stop_time - start_time)/dt)\n",
     "    time = start_time\n",
     "    for i in range(n_steps):\n",
-    "        time = timestepper.do_step(simulator, time, dt)\n",
+    "        time = do_step(timestepper, stages_and_updates, simulator, time, dt)\n",
     "    plot_timoshenko_dynamic(shearable_rod_new, unshearable_rod_new, end_force, time, ax)\n",
     "    return time\n",
     "\n",
@@ -488,7 +488,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.7"
+   "version": "3.6.4"
   }
  },
  "nbformat": 4,