fix urls in json help pages

Hello,

Url generated in json help pages are broken :
```
{{path_to_root}}json/{{ child|e }}
{{path_to_root}}json/{{ child|e }}/help
```

This generates invalid urls like. For exemple in json/builders, the url for child nodes are
http://localdomain/json/my_builder1
http://localdomain/json/my_builder2

instead of:
http://localdomain/json/builders/my_builder1
http://localdomain/json/builders/my_builder2

Fix is to make child nodes url relatives.