Update translations.json

server/translations.json

@@ -429,7 +429,7 @@
     "view_source": "Source code on GitHub",
     "zoom": "Zoom"
   },
-  "zh-cn": {
+  "zh-CN": {
     "board_name_placeholder": "白板名称",
     "circle": "圈",
     "click_to_toggle": "单击以切换",