fix r process to JSON

[jonathom]

In Cookbook Section "Output: Process as JSON"

• uses as() instead of deprecated create_user_process
• uses rjson::toJSON instead of jsonlite::toJSON, because the latter is adding [] brackets around the process_id by default, which causes errors

[m-mohr]

Thanks, I've updated the code above so that it works with jsonlite and includes the "process_graph".

In the future we can update this to simply process_json <- processToJSON(process). I've created a PR to add this functionality to the next version, see Open-EO/openeo-r-client#114

[jonathom]

Thanks!