Update casting-between-types.md

Author: kindlychung

src/doc/book/casting-between-types.md

@@ -165,10 +165,15 @@ Rust lets us:
 ```rust
 use std::mem;
 
-unsafe {
-    let a = [0u8, 0u8, 0u8, 0u8];
-
-    let b = mem::transmute::<[u8; 4], u32>(a);
+fn main() {
+    unsafe {
+        let a = [0u8, 1u8, 0u8, 0u8];
+        let b = mem::transmute::<[u8; 4], u32>(a);
+        println!("{}", b); // 256
+        // or, more concisely:
+        let c: u32 = mem::transmute(a);
+        println!("{}", c); // 256
+    }
 }
 ```