docutils 0.11 以降だとエラーになるのを修正
Merged pull-request #324, from knzm/fix-docutils-error

branch 'fix-docutils-error' of https://github.com/knzm/django-docs-ja
a lot of thanks.
Followed to a corresponding to docutils 0.11 on django's core.

refs pull-request #324
https://github.com/django/django/commit/a3a59a3197d4e03c5c016edc581bfc5f25668bca