doc/mgr: edit iostat.rst

Rewrite the first sentence in doc/mgr/iostat.rst. This follows up on a
request made by Anthony D'Atri in
ceph#63418 (comment).

Signed-off-by: Zac Dover <[email protected]>
(cherry picked from commit 766a58c)

Author: zdover23

doc/mgr/iostat.rst

@@ -3,8 +3,7 @@
 iostat
 ======
 
-The ``iostat`` module shows the current throughput and IOPS done on the Ceph
-cluster.
+The ``iostat`` module reports metrics for cluster throughpout and IOPS. 
 
 Enabling
 --------