typo in Readme: hrmp channel config

[doutv]

The sender and recipient in json config should be int type.

This example config in README does not work and produce the following error:

"hrmpChannels": [
    {
        "sender": "200",
        "recipient": "300",
        "maxCapacity": 8,
        "maxMessageSize": 512
    }
]

Error parsing spec file: invalid type: string "200"

Should be

"hrmpChannels": [
    {
        "sender": 200,
        "recipient": 300,
        "maxCapacity": 8,
        "maxMessageSize": 512
    }
]