`_fixTitle` in docs

IIRC this isn't part of the public API but we use it in docs.

https://github.com/twbs/bootstrap/blob/807e8fe4a2d955503425ec6f20fa000228089453/site/docs/4.1/assets/js/src/application.js#L78

and

https://github.com/twbs/bootstrap/blob/807e8fe4a2d955503425ec6f20fa000228089453/site/docs/4.1/assets/js/src/application.js#L95

Either we should replace it in docs or maybe make it public?

/CC @Johann-S