Docs: invalid field and invalid page

[Becheler]

Thank you for developing this resource! :)

• In the occurence module page, a R codeline seems to have made its way: fields – [str] Default (all) returns all fields. minimal returns just taxon name, key, latitude, and longitude. Or specify each field you want returned by name, e.g. fields = c('name','latitude','elevation')
  But c is not defined, and I guess should be replaced by the python equivalent ['name','latitude','elevation']
• Also, on the contributing page, the link to the issue tracker is broken.