Add lesson ending sentence

Author: uellenberg

sections/01_rust_projects/040_smiley_face/README.md

@@ -41,4 +41,6 @@ Second, you're going to need to have a list of strings for each row, and slowly
 Here are some hints to help you along:
 * Replace a character in a string: ||string.replace_range(x..=x, "*")||
 * Random number between -1 and 1: ||rng.random_range(-1..=1)||
-* Adding with type casting: ||y = (y as i64 + val) as usize||
+* Adding with type casting: ||y = (y as i64 + val) as usize||
+
+Happy coding!