save_bulk/api_views: Fix a curl command in endpoint documentation
When sending origins in CSV format to the save bulk endpoint, the text/csv content type must be explicitly specified by the HTTP client.
$ curl -X POST -H "Authorization: Bearer $SWH_TOKEN" --data-binary @erasmatazz https://archive.softwareheritage.org/api/1/origin/save/bulk/
{"exception":"UnsupportedMediaType","reason":"Unsupported media type \"application/x-www-form-urlencoded\" in request."}
$ curl -X POST -H "Authorization: Bearer $SWH_TOKEN" -H "Content-Type: text/csv" --data-binary @erasmatazz https://archive.softwareheritage.org/api/1/origin/save/bulk/
{"status":"accepted","request_id":"ef65a122-3c33-4355-8f4c-32c24fe4a232","request_info_url":"https://archive.softwareheritage.org/api/1/origin/save/bulk/request/ef65a122-3c33-4355-8f4c-32c24fe4a232/"}