Fixing code snippet for production build guide.

Author: swapnilmishra

content/guides/production-build.md

@@ -8,6 +8,7 @@ contributors:
   - simon04
   - kisnows
   - chrisVillanueva
+  - swapnilmishra
 ---
 
 This page explains how to generate production builds with webpack.
@@ -134,7 +135,7 @@ function buildConfig(env) {
   return require('./config/' + env + '.js')({ env: env })
 }
 
-module.exports = buildConfig(env);
+module.exports = buildConfig;
 ```
 And from our package.json, where we build our application using webpack, the command goes like this:
 ```js