Update cast.md

Fix comment for `128 as i8` example.

src/types/cast.md

@@ -52,7 +52,8 @@ fn main() {
 
     // Unless it already fits, of course.
     println!(" 128 as a i16 is: {}", 128 as i16);
-    // 128 as u8 -> 128, whose two's complement in eight bits is:
+    
+    // 128 as u8 -> -128, whose two's complement in eight bits is:
     println!(" 128 as a i8 is : {}", 128 as i8);
 
     // repeating the example above