doc/mgr/ceph_api: edit index.rst

doc/mgr/ceph_api: edit index.rst

Edit doc/mgr/ceph_api/index.rst as part of the project to determine
where the error is in https://github.com/ceph/ceph/pull/62782 that
prevents the Jenkins tests from passing.

This is a change to one of twenty-five files in
https://github.com/ceph/ceph/pull/62782, and this commit represents one
of what will be at least twenty-five other commits made to track this
error down.

Signed-off-by: Zac Dover <zac.dover@proton.me>