fix typo: HDR_DUPLICATE_DETECTION_ID is a field of the Message interface

The original version says the duplicate detection id string is `org.hornetq.api.core.HDR_DUPLICATE_DETECTION_ID`, which doesn't exist.
Merge pull request #1090 from baroquebobcat/patch-2

fix typo: HDR_DUPLICATE_DETECTION_ID is a field of the Message interface