fix attribution

Author: Kanahiro

src/main.ts

@@ -6,6 +6,8 @@ import { useGsiTerrainSource } from './terrain';
 const gsiTerrainSource = useGsiTerrainSource(maplibreGl.addProtocol, {
     tileUrl: 'https://tiles.gsj.jp/tiles/elev/mixed/{z}/{y}/{x}.png',
     maxzoom: 17,
+    attribution:
+        '<a href="https://gbank.gsj.jp/seamless/elev/">産総研シームレス標高タイル</a>',
 });
 
 new Map({