docs(guides): fix definition of webpack -p in production.md (#1349)

Fix definition of `webpack -p` command line option expansion, at the 
top of this page its missing the needed single quotes which are present 
down below.

Author: JonnieCache

content/guides/production.md

@@ -20,7 +20,7 @@ The following article describes the best practices and tools to use when using w
 
 ## The Automatic Way
 
-Running `webpack -p` (or equivalently `webpack --optimize-minimize --define process.env.NODE_ENV="production"`). This performs the following steps:
+Running `webpack -p` (or equivalently `webpack --optimize-minimize --define process.env.NODE_ENV="'production'"`). This performs the following steps:
 
 - Minification using `UglifyJsPlugin`
 - Runs the `LoaderOptionsPlugin` (see its [documentation](/plugins/loader-options-plugin))