add commas to waterfall example in REAMDE - thanks dbro

Author: Caolan McMahon

README.md

@@ -635,10 +635,10 @@ __Example__
     async.waterfall([
         function(callback){
             callback(null, 'one', 'two');
-        }
+        },
         function(arg1, arg2, callback){
             callback(null, 'three');
-        }
+        },
         function(arg1, callback){
             // arg1 now equals 'three'
             callback(null, 'done');