DOC: Modify ContourMeanDistanceImageFilter comments.

Author: v.fortunati@erasmusmc.nl
There is was an error in line 34. Max is replaced with mean as in the definition of the directed mean distance filter.

Change-Id: Icd7ffe394ad17ff662ee1054d82f07517279b894
Merge topic 'EditDoxygen_ContourMeanDistanceImageFilter_1358167836'

7f80d3e DOC: Modify ContourMeanDistanceImageFilter comments.