remove unused variable, update multi-instance example in readme

webpack.config.js

@@ -2,7 +2,7 @@ const path = require('path');
 
 module.exports = {
   mode: 'production',
-  entry: ["./js/index.js"],
+  entry: ['./js/index.js'],
   output: {
     path: path.join(__dirname, '/dist'),
     filename: 'reveal.min.js',
@@ -15,8 +15,8 @@ module.exports = {
         loader: 'babel-loader',
         query: {
           presets: [
-            [ '@babel/preset-env' ],
-          ],
+            [ '@babel/preset-env' ]
+          ]
         }
       }
     ]