Update configuration.md

configuration.md

@@ -45,6 +45,10 @@ ceph osd pool create ceph_metadata  2  replicated nvme_meta
 ceph osd pool set ceph_metadata size 2
 ceph osd force-create-pg 2.0 --yes-i-really-mean-it
 ```
+To change the number of placement groups in a pool
+```
+ceph osd pool set <pool name> pg_num <num>
+```
 ### Pool autotune
 ```
 ceph osd pool autoscale-status