Update example code

Author: okuryu

README.md

@@ -45,14 +45,15 @@ serialize({
     map  : new Map([['hello', 'world']]),
     set  : new Set([123, 456]),
     fn   : function echo(arg) { return arg; },
-    re   : /([^\s]+)/g
+    re   : /([^\s]+)/g,
+    big  : BigInt(10),
 });
 ```
 
 The above will produce the following string output:
 
 ```js
-'{"str":"string","num":0,"obj":{"foo":"foo"},"arr":[1,2,3],"bool":true,"nil":null,"undef":undefined,"inf":Infinity,"date":new Date("2016-04-28T22:02:17.000Z"),"map":new Map([["hello","world"]]),"set":new Set([123,456]),"fn":function echo(arg) { return arg; },"re":new RegExp("([^\\\\s]+)", "g")}'
+'{"str":"string","num":0,"obj":{"foo":"foo"},"arr":[1,2,3],"bool":true,"nil":null,"undef":undefined,"inf":Infinity,"date":new Date("2016-04-28T22:02:17.000Z"),"map":new Map([["hello","world"]]),"set":new Set([123,456]),"fn":function echo(arg) { return arg; },"re":new RegExp("([^\\\\s]+)", "g"),"big":BigInt("10")}'
 ```
 
 Note: to produced a beautified string, you can pass an optional second argument to `serialize()` to define the number of spaces to be used for the indentation.