added function mutable by reference

Author: zeandcode

stage-5/function_reference_mutable.rs

@@ -0,0 +1,9 @@
+fn modifies(x: &mut f64) {
+    *x = 1.0
+}
+
+fn main() {
+    let mut val = 18.5;
+    modifies(&mut val);
+    println!("{}", val);
+}