Update vietnamese icon

Author: viettel-solutions

README.md

@@ -1,7 +1,7 @@
 [🇨🇳](/README-cn.md "Simplified Chinese")
 [🇰🇷](/README-ko.md "Korean")
 [🇪🇸](/README-es.md "Spanish")
-[vn](/README-vn.md "Vietnamese")
+[:vietnam:](/README-vn.md "Vietnamese")
 
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