Update example. (#13)

Author: happy-san

README.md

@@ -10,15 +10,15 @@ A simple usage example:
 ```dart
 import 'package:bit_array/bit_array.dart';
 
-main() {
-  final array = new FixedBitArray(1024);
+void main() {
+  final array = BitArray(1024);
   array[12] = true;
   array.setBit(123);
   array.invertBit(200);
   array.clearBit(12);
   print(array.asIntIterable().toList()); // prints [123, 200]
 
-  final other = new FixedBitArray(1024);
+  final other = BitArray(1024);
   other[123] = true;
 
   final and = array & other;