Fix typo

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1570585 13f79535-47bb-0310-9956-ffa450edef68
Add more clarification re MBean interfaces.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1570586 13f79535-47bb-0310-9956-ffa450edef68