docs: Update README.md

Author: hsluoyz

README.md

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-Xorm Adapter
+# Xorm Adapter
+
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----
 
 Xorm Adapter is the [Xorm](https://gitea.com/xorm/xorm) adapter for [Casbin](https://github.com/casbin/casbin). With this library, Casbin can load policy from Xorm supported database or save policy to it.