Fix Rails Integration docs: Missing module export statement (#263)

Thanks for this great library @claviska! 

Tried adding Shoelace to a Rails app w/ the current guide. When starting the Webpack Dev Server, the following error gets thrown:

```
.../RailsApplication/config/webpack/development.js:5
module.exports = environment.toWebpackConfig()
```
Adding `module.exports = environment` as done [here in the example repo](4114da0e14/config/webpack/environment.js (L20)) fixes the issue and allows Webpack to compile.

docs/tutorials/integrating-with-rails.md

@@ -69,6 +69,8 @@ environment.plugins.append(
     ]
   })
 )
+
+module.exports = environment
 ```
 
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