clarified spec

dlib/image_processing/scan_fhog_pyramid_abstract.h

@@ -604,7 +604,7 @@ namespace dlib
             ensures
                 - This function allows you to determine the feature vector used for an
                   object detection output from detect().  Note that this vector is
-                  added to psi.  Note also that you must use get_full_object_detection() to
+                  added to psi.  Note also that you can use get_full_object_detection() to
                   convert a rectangle from detect() into the needed full_object_detection.
                 - The dimensionality of the vector added to psi is get_num_dimensions().  This
                   means that elements of psi after psi(get_num_dimensions()-1) are not modified.