fix typo

不加return会继续往下执行resolve

docs/async.md

@@ -574,7 +574,7 @@ var fs = require('fs');
 var readFile = function (fileName){
   return new Promise(function (resolve, reject){
     fs.readFile(fileName, function(error, data){
-      if (error) reject(error);
+      if (error) return reject(error);
       resolve(data);
     });
   });