Fix unfortunate typo generated by old sphinx-doc (#2020) in to rare dictionary

A lot of repositories contain this typo which was introduced by a bug in
a sphinx-doc template which has since been fixed. Add correction to rare dictionary.

Examples:
* https://github.com/mozillazg/python-pinyin/pull/196
* https://github.com/lambdalisue/django-permission/pull/84
* https://github.com/philippbosch/django-geoposition/pull/113
* https://github.com/revsys/django-test-plus/pull/117
* https://github.com/danjac/pyramid_storage/pull/38
* https://github.com/tetframework/Tonnikala/pull/7`
* https://github.com/duerrp/pyexperiment/pull/2