Print the output path when saving a screenshot

Quality-of-life change; A user does not need to check the source code or
look for a file in a messy directory. The path is subject to change,
though (probably as a CLI parameter)

main.c

@@ -133,5 +133,5 @@ int main() {
   // Thanks to endianess shenanigans, ABGR/XBGR is RGBA. 
   write_rif_little("out.rif", image_width, buf_size, RIF_FORMAT_R8G8B8A8, shm_data);
 
-  printf("Saved screenshot.\n");
+  printf("Saved screenshot to ./out.rif.\n");
 }