remove exclamation mark from string-program

refcell-program/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "refcell-program"
+version = "0.1.0"
+edition = "2021"

refcell-program/src/main.rs

@@ -0,0 +1,9 @@
+use std::cell::RefCell;
+
+fn main() {
+    let refcell_str: RefCell<&str> = RefCell::new("hello");
+
+    println!("refcell_str");
+    let second = refcell_str.clone();
+    println!("refcell_str second:");
+}