docs: improve snippet in README (#184)

README.md

diff --git a/README.md b/README.md
index 2a63aad..388b859 100644
--- a/README.md
+++ b/README.md
@@ -322,7 +322,7 @@
   ]) : super(year, month, day, hour, minute, second, millisecond, microsecond);
 
   @override
-  List<Object> get props => super.props..addAll([century]);
+  List<Object> get props => [...super.props, century];
 }
 ```