gh-37328: remove "known bug" marker for #6413 from doctest
The long-standing issue #6413 was resolved in #37096, but there is still
one `# known bug` marker for it in
`src/sage/schemes/elliptic_curves/hom_scalar.py`. Let's remove it.
(Incidentally, the claimed output for the invocation that used to not
work was slightly wrong, too.)
URL: https://github.com/sagemath/sage/pull/37328
Reported by: Lorenz Panny
Reviewer(s):
