Merge pull request expressjs#12 from trevordixon/patch-1

Fix README typo.

README.md

@@ -91,7 +91,7 @@ var whitelist = ['http://example1.com', 'http://example2.com'];
 var corsOptions = {
   origin: function(origin, callback){
     var originIsWhitelisted = whitelist.indexOf(origin) !== -1;
-    cb(null, originIsWhitelisted);
+    callback(null, originIsWhitelisted);
   }
 };