finish using_as exercise

exercises/conversions/using_as.rs

@@ -2,16 +2,13 @@
 // Please note that the `as` operator is not only used when type casting.
 // It also helps with renaming imports.
 
-// I AM NOT DONE
 // The goal is to make sure that the division does not fail to compile
 fn average(values: &[f64]) -> f64 {
-    let total = values
-        .iter()
-        .fold(0.0, |a, b| a + b);
-    total / values.len()
+    let total = values.iter().fold(0.0, |a, b| a + b);
+    total / (values.len() as f64)
 }
 
 fn main() {
     let values = [3.5, 0.3, 13.0, 11.7];
     println!("{}", average(&values));
-}
+}