[Fix Bug] cannot render corr_vars's valueEnum correctly.

resources/templates/plugin-schema.handlebars.json

       "valueType": "select",
       "title": "Correlation Variables",
       "tooltip": "Which variables do you want to analyze?",
-      "valueEnum": "#{{{tojson columns }}}#",
+      "valueEnum": "#{{{toselect columns }}}#",
       "fieldProps": {
         "mode": "multiple"
       },