add a greek flag link to the HISTORY.md for the HISTORY_GR.md translated document
-# History/Changelog <a href="HISTORY_ZH.md"> <img width="20px" src="https://iris-go.com/images/flag-china.svg?v=10" /></a>
+# History/Changelog <a href="HISTORY_ZH.md"> <img width="20px" src="https://iris-go.com/images/flag-china.svg?v=10" /></a> <a href="HISTORY_GR.md"> <img width="20px" src="https://iris-go.com/images/flag-greece.svg?v=10" /></a>
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