GNotification: Some small documentation fixes

Use #GVariant instead of GVariant.

g_notification_add_button_with_target,
g_notification_set_default_action_and_target:
Replace 'format_string' with 'target_format'.

g_notification_set_default_action_and_target_value:
Remove paragraph that apparently had been accidentally copied from
g_notification_set_default_action_and_target.

https://bugzilla.gnome.org/show_bug.cgi?id=727123