Update use-sync-waitgroup-to-synchronize-goroutines.md.

Author: NanXiao

posts/use-sync-waitgroup-to-synchronize-goroutines.md

@@ -1,5 +1,7 @@
 # Use sync.WaitGroup to synchronize goroutines
 ----
+(This post is a modification edition of [Use sync.WaitGroup in Golang](http://nanxiao.me/en/use-sync-waitgroup-in-golang/)).  
+
 [sync.WaitGroup](https://golang.org/pkg/sync/#WaitGroup) provides a goroutine synchronization mechanism, and used for waiting for a collection of goroutines to finish. In the internal of `sync.WaitGroup` struct, there is a `counter` which records how many goroutines need to be waited are living now.  
 
 `sync.WaitGroup` provides `3` methods: `Add`, `Done` and `Wait`. `Add` method is used to identify how many goroutines need to be waited, and it will add `counter` value. When a goroutine exits, it must call `Done`, and it will decrease `counter` value by `1`. The `main` goroutine blocks on `Wait`, once the `counter` becomes `0`, the `Wait` will return, and main goroutine can continue to run.
@@ -35,7 +37,4 @@ Because the `main` goroutine need to wait `2` goroutines, so the argument for `w
 	goroutine exit
 	goroutine exit
 	Main goroutine exit
-Please notice, the `Add` must go ahead of `Done`.  
-
-Reference:  
-[Use sync.WaitGroup in Golang](http://nanxiao.me/en/use-sync-waitgroup-in-golang/).
+