doc/mgr: edit alerts.rst

doc/mgr: edit alerts.rst

Edit doc/mgr/alerts.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 commit adds to the work done in
https://github.com/ceph/ceph/pull/62782 by correcting some of the
English that was present in that PR.

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>