example: a reminder that usize is assumed to be 8 bytes long (#188)

examples/basic.rs

@@ -18,7 +18,7 @@ fn main() {
 
     let encoded: Vec<u8> = serialize(&world, Infinite).unwrap();
 
-    // 8 bytes for the length of the vector, 4 bytes per float.
+    // 8 bytes for the length of the vector (usize), 4 bytes per float.
     assert_eq!(encoded.len(), 8 + 4 * 4);
 
     let decoded: World = deserialize(&encoded[..]).unwrap();