STEXI and ETEXI is not used anymore since we switched to Sphinx.
Replace them in the example with SRST and ERST, too.

Message-Id: <20220506150146.564244-1-thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>