docs(guides): fix source-map discrepancy in production.md (#1711)

The `webpack.prod.js` code example in the "Specify the Evironment"
section shows `cheap-module-source-map` for the `devtool` field which is
not recommended for production. This section should show `source-map`
to be consistent with the webpack.prod.js in the previous
"Source Mapping" section.

Author: mbsrc

src/content/guides/production.md

@@ -195,7 +195,7 @@ __webpack.prod.js__
   const common = require('./webpack.common.js');
 
   module.exports = merge(common, {
-    devtool: 'cheap-module-source-map',
+    devtool: 'source-map',
     plugins: [
       new UglifyJSPlugin({
         sourceMap: true