fix: add 'mode' property in second array object

Resolve issue in examples/general/config-array where absence of 'mode'
property in second configuration object caused an error.

Fixes #5085

Author: ayushkumarbhadani

examples/general/config-array/webpack.config.js

@@ -34,6 +34,7 @@ module.exports = [
     output: {
       filename: "bundle2.js",
     },
+    mode: "development",
     module: {
       rules: [
         {