Extra newline in exception

```
Sprockets::Rails::Helper::AssetNotPrecompiled: Asset `application.scss` was not declared to be precompiled in production.
Declare links to your assets in `app/assets/config/manifest.js`.

  //= link application.scss
and restart your server
```

Becomes

```
Sprockets::Rails::Helper::AssetNotPrecompiled: Asset `application.scss` was not declared to be precompiled in production.
Declare links to your assets in `app/assets/config/manifest.js`.

  //= link application.scss

and restart your server
```

lib/sprockets/rails/helper.rb

@@ -16,7 +16,7 @@ def initialize(source)
           if using_sprockets4?
             "Asset `#{ source }` was not declared to be precompiled in production.\n" +
             "Declare links to your assets in `app/assets/config/manifest.js`.\n\n" +
-            "  //= link #{ source }\n" +
+            "  //= link #{ source }\n\n" +
             "and restart your server"
           else
             "Asset was not declared to be precompiled in production.\n" +