added missing requirement.

dlib/svm/structural_svm_object_detection_problem_abstract.h

@@ -88,6 +88,7 @@ namespace dlib
             requires
                 - is_learning_problem(images, truth_rects)
                 - scanner.get_num_detection_templates() > 0
+                - scanner.load(images[0]) must be a valid expression.
             ensures
                 - This object attempts to learn a mapping from the given images to the 
                   object locations given in truth_rects.  In particular, it attempts to