copy(search): shorten mobile search placeholder

src/i18n.tsx

@@ -21,7 +21,7 @@ const zhDict: Dict = {
   popular: "热门资料",
   search: "搜索",
   searchPlaceholder: "搜索资料...",
-  searchPanelPlaceholder: "搜索 PPT、影片、海报、公告、教程、文...",
+  searchPanelPlaceholder: "搜索资料...",
   searchNow: "立即搜索资料",
   searchSubmit: "搜索",
   cancel: "取消",
@@ -148,7 +148,7 @@ const enDict: Dict = {
   popular: "Popular",
   search: "Search",
   searchPlaceholder: "Search resources...",
-  searchPanelPlaceholder: "Search PPT, videos, posters, news, guides...",
+  searchPanelPlaceholder: "Search assets...",
   searchNow: "Search now",
   searchSubmit: "Search",
   cancel: "Cancel",