Fix "url" in extension schema

[balazsdukai]

It was missed by 87720c17878495f927a87889407f675e91e60635

However, the uri needs to stay in the validation in order to preserve compatibility with v1.1.

"anyOf": [
    {
      "required": [
        "url"
      ]
    },
    {
      "required": [
        "uri"
      ]
    }
  ],