Update Vietnamese page link

Author: hnq90

index.html

@@ -47,6 +47,7 @@ <h1>git - the simple guide</h1>
             <a href="index.ja.html">日本語</a>, 
             <a href="index.zh.html">中文</a>, 
             <a href="index.ko.html">한국어</a>
+            <a href="index.vi.html">Vietnamese</a>
             <br />
             please report issues on <a href="https://github.com/rogerdudler/git-guide/issues">github</a>
         </p>