Fix building guides outside ./guides directory

As is the case with the `rake preview_docs` task.

```
Error reading assets/stylesrc/style.scss: Cannot open file.

Error reading assets/stylesrc/highlight.scss: Cannot open file.

Error reading assets/stylesrc/print.scss: Cannot open file.
```

Author: zzak

guides/rails_guides/generator.rb

@@ -107,7 +107,10 @@ def select_only(guides)
       end
 
       def process_scss
-        system "bundle exec dartsass ./assets/stylesrc/style.scss:#{@output_dir}/stylesheets/style.css ./assets/stylesrc/highlight.scss:#{@output_dir}/stylesheets/highlight.css ./assets/stylesrc/print.scss:#{@output_dir}/stylesheets/print.css"
+        system "bundle exec dartsass \
+          #{@guides_dir}/assets/stylesrc/style.scss:#{@output_dir}/stylesheets/style.css \
+          #{@guides_dir}/assets/stylesrc/highlight.scss:#{@output_dir}/stylesheets/highlight.css \
+          #{@guides_dir}/assets/stylesrc/print.scss:#{@output_dir}/stylesheets/print.css"
       end
 
       def copy_assets