HADOOP-9350: move entry in changes.txt to 2.1.2-beta section

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1523191 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-271-trunk-gd