Fix guide section about assets precompiling task in production [ci skip]

You need to specify `RAILS_ENV=production`

Related to https://github.com/rails/sass-rails/issues/152.

/cc @guilleiguaran

guides/source/asset_pipeline.md

@@ -416,7 +416,7 @@ You can call this task on the server during deployment to create compiled versio
 The rake task is:
 
 ```bash
-$ bundle exec rake assets:precompile
+$ RAILS_ENV=production bundle exec rake assets:precompile
 ```
 
 For faster asset precompiles, you can partially load your application by setting