Changed preview code

assets/hello.rs

@@ -1,6 +0,0 @@
-
-// Hello World
-fn main() {
-    println!("Hello World!");
-}
-

assets/theme_preview.rs

@@ -0,0 +1,5 @@
+// Output the square of a number.
+fn print_square(num: f64) {
+    let result = f64::powf(num, 2.0);
+    println!("The square of {:.2} is {:.2}.", num, result);
+}