Move HDFS-8923 to Release 2.8.0 in CHANGES.txt.

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -918,6 +918,9 @@ Release 2.8.0 - UNRELEASED
     HDFS-9773. Remove dead code related to SimulatedFSDataset in tests.
     (Brahma Reddy Battula via aajisaka)
 
+    HDFS-8923. Add -source flag to balancer usage message.  (Chris Trezzo
+    via szetszwo)
+
   OPTIMIZATIONS
 
     HDFS-8026. Trace FSOutputSummer#writeChecksumChunks rather than
@@ -1806,9 +1809,6 @@ Release 2.7.3 - UNRELEASED
     HDFS-4946. Allow preferLocalNode in BlockPlacementPolicyDefault to be
     configurable (James Kinley and Nathan Roberts via kihwal)
 
-    HDFS-8923. Add -source flag to balancer usage message.  (Chris Trezzo
-    via szetszwo)
-
   OPTIMIZATIONS
 
   BUG FIXES