Update index.asciidoc

In some occasions you may get an HTTP 404 error when you try to delete a river. 
{"error":"TypeMissingException[[_all] type[[test]] missing: No index has the type.]","status":404}

In this case you need to supply JSON data with your HTTP DELETE request
curl -XDELETE 'localhost:9200/_river/my_river/' -d '{"type": "dummy"}'