Help for a Playgrounds subscription

I wanted to try to open my feed for a playgroiunds subscription. I followed instructions in the video of WWWDC18. I used GitHub as a repository.

My site appears at address https: // giobus.github.io.

But if you follow the 'subscribe' link it will not work. Also if you insert directly the subscription link.

The error is "Verify the url and retry", I checked everything, everything seems fine, but obviously I'm wrong. If I use the direct link ( giobus.github.io/ordiniamo/ordiniamo.playgroundbook.zip) on github to download the playground it works !!!!

Also the .json file looks fine, I checked it with file in the video WWWDC18 and I can not see any difference.

I do not understand why ... any help!?
This my .json file, I wrote it with "Sublime text"

{

"title": "Giobus Playgrounds",

"publisherName": "Giovanna Busconi",

"feedIdentifier" : "io.github.giobus",

"contactURL" : "mailto:giovanna.busconi@gmail.com",

"formatversion": "1.1",

"documents": [

{

"title" : "Ordiniamo",

"overviewSubtitle" : "Ragionanado sull'ordinamento di vettori",

"detailSubtitle": "Ordinamento Bubblesort e Per scambio",

"description": "Playground realizzato durante un corso di formazione docenti",

"contentIdentifier": "io.github.giobus.ordiniamo",

"contentVersion": "1.0",

"url": "https://giobus.github.io/ordiniamo/ordiniamo.playgroundbook.zip",

"publishedDate": "2018-07-09T09:00:00+07:00",

"lastUpdateDate" : "2018-07-10T12:00:00+07:00",

"thumbnailURL": "https://giobus.github.io/ordiniamo/thumbnail.png",

"bannerImageURL" : "https://giobus.github.io/ordiniamo/bannerImage.png",

"additionalInformation" : [

{

"name": "Pensato per",

"value": "scuola secondaria secondo grado"

}

],

"previewImageURLs" : [ ]

}

]

}