[Docs] Fix incorrect Pub/Sub example resulting in BadRequestException

[adriankirchner]

Fix incorrect Pub/Sub example resulting in BadRequestException:

Fatal error: Uncaught Google\Cloud\Core\Exception\BadRequestException: {
  "error": {
    "code": 400,
    "message": "Invalid value at 'messages[0].attributes[0].value' (TYPE_STRING), 1",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "field": "messages[0].attributes[0].value",
            "description": "Invalid value at 'messages[0].attributes[0].value' (TYPE_STRING), 1"
          }
        ]
      }
    ]
  }
}

As documented [1] attributes must look like this map (key: string, value: string)

[1] https://cloud.google.com/pubsub/docs/reference/rest/v1/PubsubMessage

[dwsupplee]

Looks great, thanks @adriankirchner!