Change "<id>" to "<subscription id>" in NIP-01

[vinliao]

In NIP-01, there's a "REQ" message from client to relay, which looks like this: ["REQ", <id>, <filter JSON>...]. The <id> part confused me a bit (took me some time to figure out what it is). I thought it was event id.

I think it would be less confusing if it's renamed to <subscription id> or <subscription string>. Not a critical issue; just a semantic change to make it easier to understand.

[scsibug]

Agreed, it would be good to disambiguate from the event id.