eoc7: show elapsed time out to milliseconds.

rtchallenge/examples/eoc7.rs

@@ -91,6 +91,6 @@ fn main() -> Result<()> {
     let path = "/tmp/eoc7.png";
     println!("saving output to {}", path);
     image.write_to_file(path)?;
-    println!("Render time {:.2} seconds", start.elapsed().as_secs_f32());
+    println!("Render time {:.3} seconds", start.elapsed().as_secs_f32());
     Ok(())
 }