chore: update license in plugin.yaml (#8)

更新 plugin.yaml 中的 License 类型，修复与仓库的 License 文件不对应的问题。

/kind improvement

```release-note
None
```

Author: ruibaby

src/main/resources/plugin.yaml

@@ -14,4 +14,5 @@ spec:
   displayName: "Sitemap"
   description: "为站点生成站点地图"
   license:
-    - name: "MIT"
+    - name: "GPL-3.0"
+      url: "https://github.com/halo-sigs/plugin-sitemap/blob/main/LICENSE"