gh-37233: fix small mistake in BQFClassGroupQuotientMorphism documentation
The possibility to pass discriminants to the constructor instead of
`BQFClassGroup` objects was present for an earlier version of #37074,
but it was removed during review. However, it seems I forgot to update
the docstring accordingly.
URL: https://github.com/sagemath/sage/pull/37233
Reported by: Lorenz Panny
Reviewer(s): Travis Scrimshaw