Add call_me function to print "Hello World!"

exercises/02_functions/functions1.rs

@@ -3,7 +3,9 @@
 // Execute `rustlings hint functions1` or use the `hint` watch subcommand for a
 // hint.
 
-// I AM NOT DONE
+fn call_me() {
+    println!("Hello World!");
+}
 
 fn main() {
     call_me();