minor doc bug for Gtk2::MessageDialog

[Dave Howorth <dhoworth mrc-lmb cam ac uk>]

I believe there's a line missing at the end of the example code listed
in the SYNOPSIS section for Gtk2::MessageDialog:

  $dialog->show_all;

The example code on the Gtk2::Dialog page is complete. It appears that
this error was inherited from the gtk+ docs.

Cheers, Dave