Tutorial uses deprecated method

[amindfv at mailbox.org]

In the latest git version of Basic Tutorial 5 (gst-docs/examples/tutorials/basic-tutorial-5.c), we get a warning about using a deprecated method:

    basic-tutorial-5.c: In function ‘create_ui’:
    basic-tutorial-5.c:130:3: warning: ‘gtk_widget_set_double_buffered’ is deprecated [-Wdeprecated-declarations]
       gtk_widget_set_double_buffered (video_window, FALSE);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tom