Reference the `get_url` method in Page model reference docs

Fixes #4424. As of #3354, `get_url` is the preferred way of obtaining a page URL, rather than the `.url` property;
.url is just a wrapper around get_url (which meant that the docstring for `get_url` was erroneously being picked up).

docs/reference/pages/model_reference.rst

@@ -117,7 +117,7 @@ In addition to the model fields provided, ``Page`` has many properties and metho
 
     .. autoattribute:: specific_class
 
-    .. autoattribute:: url
+    .. automethod:: get_url
 
     .. autoattribute:: full_url