examples/muxing: cast sws_scale() argument to the expected one

Fix warnings:
muxing.c: In function ‘write_video_frame’:
muxing.c:326:23: warning: passing argument 2 of ‘sws_scale’ from incompatible pointer type [enabled by default]

doc/examples/muxing.c

@@ -340,7 +340,8 @@ static void write_video_frame(AVFormatContext *oc, AVStream *st)
                 }
             }
             fill_yuv_image(tmp_picture, frame_count, c->width, c->height);
-            sws_scale(img_convert_ctx, tmp_picture->data, tmp_picture->linesize,
+            sws_scale(img_convert_ctx,
+                      (const uint8_t * const *)tmp_picture->data, tmp_picture->linesize,
                       0, c->height, picture->data, picture->linesize);
         } else {
             fill_yuv_image(picture, frame_count, c->width, c->height);