more precise description for --function option

src/seam-carving.cpp

@@ -520,7 +520,7 @@ int main(int argc, char **argv) {
   app.add_flag("--show-seams", show_seams,
                "Don't resize image, just try the specified energy function");
   app.add_option("-f,--function", function,
-                 "The function to apply to compute the energy");
+                 "The function to apply to compute the energy (default: gradient)");
   app.add_flag(
       "-u,--until-mask-removal", until_mask_removal,
       "Carve the image until there are no more red pixels in the mask");