Justify "4".

Author: ben0x539

src/expressions/operator-expr.md

@@ -404,7 +404,7 @@ A trivial example of sound address arithmetic:
 let mut values: [i32; 2] = [1, 2];
 let p1: *mut i32 = values.as_mut_ptr();
 let first_address = p1 as usize;
-let second_address = first_address + 4;
+let second_address = first_address + 4; // 4 == size_of::<i32>()
 let p2 = second_address as *mut i32;
 unsafe {
     *p2 += 1;