fix in Building for Production docs

change ` loader: 'file'`
to `loader: 'file-loader'`

Author: aryzle

content/guides/production-build.md

@@ -177,7 +177,7 @@ module.exports = function() {
                 loaders: ['to-string-loader', 'css-loader']
             }, {
                 test: /\.(jpg|png|gif)$/,
-                loader: 'file'
+                loader: 'file-loader'
             }, {
                 test: /\.(woff|woff2|eot|ttf|svg)$/,
                 loader: 'url-loader?limit=100000'