tails-documentation: use a proper icon for the back button.

config/chroot_local-includes/usr/local/bin/tails-documentation

@@ -29,7 +29,7 @@ class DocumentationWindow(tailsgreeter.gui.GreeterHelpWindow):
         # variable, but we need it!
         headerbar = next(child for child in self.get_children() \
                          if isinstance(child, Gtk.HeaderBar))
-        back_button = Gtk.Button('←')
+        back_button = Gtk.Button.new_from_icon_name('back', Gtk.IconSize.BUTTON)
         back_button.connect("clicked", lambda x: self.webview.go_back())
         headerbar.pack_start(back_button)
         back_button.show()