fix link to djangodoc for template inheritance
The old django 1.9 doc is no longer available.
//! Every time I look into a templating system, I will investigate its
//! support for [template inheritance][t].
//!
-//! [t]: https://docs.djangoproject.com/en/1.9/ref/templates/language/#template-inheritance
+//! [t]: https://docs.djangoproject.com/en/3.2/ref/templates/language/#template-inheritance
//!
//! Template include is not sufficient for template reuse. In most cases
//! you will need a skeleton of page as parent (header, footer, etc.), and
