Add solutions to functions

solutions/02_functions/functions3.rs

@@ -1 +1,10 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn call_me(num: u32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
+
+fn main() {
+    // `call_me` expects an argument.
+    call_me(5);
+}