fix: switch star history chart to a working provider

The star history chart is currently broken because the previous provider is affected by the GitHub stargazer API restrictions. This change points the chart to a working provider so the chart is displayed correctly again in the README.

Author: OctoBored

README.md

@@ -221,7 +221,7 @@ This project exists thanks to all the people who contribute.
 
 ## Star History
 
-[![Star History Chart](https://api.star-history.com/svg?repos=apache/casbin-rs&type=Date)](https://star-history.com/#apache/casbin-rs&Date)
+[![Star History Chart](https://star-history.dera.page/svg?repos=apache/casbin-rs&type=Date)](https://star-history.dera.page/#apache/casbin-rs&Date)
 
 ## License