Update char-bool.md

`std::mem::size_of_val(&x))` can be simplified to `std::mem::`

src/basic/base-type/char-bool.md

@@ -24,7 +24,7 @@ fn main() {
 ```rust
 fn main() {
     let x = '中';
-    println!("字符'中'占用了{}字节的内存大小",std::mem::size_of_val(&x));
+    println!("字符'中'占用了{}字节的内存大小", size_of_val(&x));
 }
 ```