doc/cephadm: fix typo and missing quote in activate-existing-osds

Correct "clea" to "clear" and add the missing closing quotation mark
after the description of an "online" host in the OSD activation section.

Fixes: https://tracker.ceph.com/issues/77075
Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>