Change default.yml for link warning

Also to test whether rendering context system can refer to yml instead of json file.

Author: Kraig Brockschmidt

docs/python/default.yml

@@ -1,3 +1,3 @@
 ### YamlMime:ContextObject
-breadcrumb_path: ./_breadcrumb/toc.json
+breadcrumb_path: ./_breadcrumb/toc.yml
 toc_rel: ./toc.yml