Update copyright year to 2026

Author: mgao97

README.md

@@ -1,7 +1,7 @@
 EasyGraph
 ==================
 
-Copyright (C) <2020-2025> by [DataNET Group, Fudan University](https://fudan-datanet.mysxl.cn/)
+Copyright (C) <2020-2026> by [DataNET Group, Fudan University](https://fudan-datanet.mysxl.cn/)
 
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