DOC: Modify JointHistogramMutualInformationImageToImageMetricv4 comments.

Author: dr.tim.allman@gmail.com
Corrected comment.

Change-Id: I9115ce0a3784319db2cc417a5537f9112490ab12
Merge topic 'EditDoxygen_JointHistogramMutualInformationImageToImageMetricv4_1363190160'

d407201 DOC: Modify JointHistogramMutualInformationImageToImageMetricv4 comments.