DOC: Modify JointHistogramMutualInformationImageToImageMetricv4 comments.

Author: dr.tim.allman@gmail.com
Incorrect class name in doxygen comment.

Change-Id: Iabc5a51acaadd9c3a1d8468e654c0c6a15666667
Merge topic 'EditDoxygen_JointHistogramMutualInformationImageToImageMetricv4_1363189266'

551d338 DOC: Modify JointHistogramMutualInformationImageToImageMetricv4 comments.