doc/various: update link to CRUSH pdf
This commit updates link to the research paper that announces and
explains the CRUSH algorithm. This link was broken in the migration from
the old Ceph website to ceph.io.
Signed-off-by: Zac Dover <zac.dover@gmail.com>
(cherry picked from commit
6877b592a289cb473f16daa79d5112f5b2d8006c)