DOC: Multiple Label LaTeX Warning

Label sec:BSplinesMultiGridImageRegistration was defined twice in
two sections with identical headings. Labels corrected, headings
corrected, example in DeformableRegistartion7.cxx corrected:
http://review.source.kitware.com/#/c/13813/.

Change-Id: I1ba2dc2242b36a8447f2abbb48ff1967dbb5cd11