Update string.md

solutions/compound-types/string.md

@@ -16,6 +16,16 @@ fn main() {
      println!("{}",s)
  }
 ```
+```rust
+fn main() {
+    let s: Box<&str> = "hello, world".into();
+    greetings(*s)
+}
+
+fn greetings(s: &str) {
+    println!("{}", s);
+}
+```
 
 3. 
 ```rust