CLN: Replace incorrect GH#5667 with GH#5567
Hello!

*The checklist is not applicable here*

### Pull request overview
* Replacing `GH#5667` ina comment with `GH#5567`.

### Details
The PR #6364 closes the issue #5567, but the comment this PR introduces in `indexing.py` incorrectly refers to 5<ins>6</ins>67 instead. This code was later moved, cleaned up, etc., but the reference to the incorrect 5667 stuck around.

* Tom Aarsen
thx @tomaarsen 