use django-admin naming instead of django internal djadmin

Author: bckohan

README.rst

@@ -43,7 +43,7 @@ Improvements for the output of Sphinx's autodoc for Django classes:
 * Add information about autogenerated methods
 * Fix intersphinx mappings to Django modules
 * Custom text roles to cross-reference the documentations of Django (``:setting:``,
-  ``:templatetag:``, ``:templatefilter:``, ``:fieldlookup:``, ``:djadmin:``) and Sphinx (``:event:``,
+  ``:templatetag:``, ``:templatefilter:``, ``:fieldlookup:``, ``:django-admin:``) and Sphinx (``:event:``,
   ``:confval:``)