doc/glossary: s/OMAP/omap/

Change "OMAP" to "omap" to match the capitalization established by
Eleanor Cawthon in her 2012 omap paper, here:
https://ceph.io/assets/pdfs/CawthonKeyValueStore.pdf.

Signed-off-by: Zac Dover <[email protected]>
(cherry picked from commit 2872986)

Author: zdover23

doc/glossary.rst

@@ -344,13 +344,13 @@
 	Object Storage Device
                 See :term:`OSD`.
 
-        OMAP
+        omap 
                 "object map". A key-value store (a database) that is used to
                 reduce the time it takes to read data from and to write to the
-                Ceph cluster. RGW bucket indexes are stored as OMAPs.
-                Erasure-coded pools cannot store RADOS OMAP data structures.
+                Ceph cluster. RGW bucket indexes are stored as omaps.
+                Erasure-coded pools cannot store RADOS omap data structures.
 
-                Run the command ``ceph osd df`` to see your OMAPs.
+                Run the command ``ceph osd df`` to see your omap.
 
                 See Eleanor Cawthon's 2012 paper `A Distributed Key-Value Store
                 using Ceph