ui: fix search by tag

packages/ui-default/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@hydrooj/ui-default",
-  "version": "4.13.11",
+  "version": "4.13.12",
   "author": "undefined <i@undefined.moe>",
   "license": "AGPL-3.0",
   "main": "hydro.js",

packages/ui-default/pages/problem_main.page.js

@@ -56,7 +56,7 @@ async function updateSelection(sendRequest = true) {
       url = substitute(decodeURIComponent(UiContext.getProblemUrlWithCategory), {
         category: requestTags
           .map((tag) => tag.split(',').map(encodeURIComponent).join(','))
-          .join('+'), // build a beautiful URL
+          .join(','), // build a beautiful URL
       });
     }
     const q = $('[name="q"]').val();