fix(npm/react): webpack config sample bug (#16737)

Co-authored-by: Lachlan Miller <lachlan.miller.1990@outlook.com>

npm/react/docs/recipes.md

@@ -59,9 +59,15 @@ If you have your own Webpack config, you can use included plugins file to load i
 ```js
 // cypress/plugins/index.js
 module.exports = (on, config) => {
-  // from the root of the project (folder with cypress.json file)
-  config.env.webpackFilename = 'webpack.config.js'
-  require('@cypress/react/plugins/load-webpack')(on, config)
+  // the path is relative to cypress.json
+  const webpackFilename = './src/webpack.config.js'
+
+  // configuration
+  config.env.webpackFilename = webpackFilename
+  config.env.coverage = false
+
+  require('@cypress/react/plugins/load-webpack')(on, config, {webpackFilename})
+
   // IMPORTANT to return the config object
   // with the any changed environment variables
   return config