fix logo URL

README.md

-<a href="#"><img alt="rex" src="https://raw.githubusercontent.com/go-rex/rex/assets/images/rex.png" width="160px" height="64px"></a>
+<a href="#"><img alt="rex" src="https://raw.githubusercontent.com/goanywhere/rex/assets/images/rex.png" width="160px" height="64px"></a>
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