txn claim example values should be strings

[TakahikoKawasaki]

The SSF specification includes txn claim example values like below. Their values are integers.

{
  "jti": "123456",
  "iss": "https://transmitter.example.com",
  "aud": ["receiver.example.com/web", "receiver.example.com/mobile"],
  "iat": 1493856000,
  "txn": 8675309,
  "sub_id": {
    "format": "opaque",
    "id": "72e6991badb44e08a69672960053b342"
  },
  "events": {
    "https://schemas.openid.net/secevent/ssf/event-type/verification": {
      "state": "VGhpcyBpcyBhbiBleGFtcGxlIHN0YXRlIHZhbHVlLgo="
    }
  }
}

However, since Section 2.2. Core SET Claims of RFC 8417: Security Event Token (SET), a family spec of SSF, explains the txn claim as "An OPTIONAL string value" as below:

"txn" (Transaction Identifier) Claim

An OPTIONAL string value that represents a unique transaction identifier. In cases in which multiple related JWTs are issued, the transaction identifier claim can be used to correlate these related JWTs. Note that this claim can be used in JWTs that are SETs and also in JWTs using non-SET profiles.

txn claim example values in the SSF specification should be changed from integers to strings.