Changed {request>user_id} to {user_id}

[steffenbusch]

The {request>user_id} used in the README of this plugin as well as in the definition of commonLogFormat is not returning the value that represents "user_id": "foobar" in the JSON access logs / the value of the placeholder http.auth.user.id.

I am not sure why {request>user_id} was used, but {user_id} works for me in case of basic_auth usage or using the new functionality described here caddyserver/caddy#6108

For me, {user_id} instead of {request>user_id} makes sense because the user_id is on the same level as ts ({ts}), status ( {status}) or size ({size}) as described in https://caddyserver.com/docs/logging#structured-logs

{
	"level": "info",
	"ts": 1646861401.5241024,
	"logger": "http.log.access",
	"msg": "handled request",
	"request": {
		"remote_ip": "127.0.0.1",
		"remote_port": "41342",
		"client_ip": "127.0.0.1",
		"proto": "HTTP/2.0",
		"method": "GET",
		"host": "localhost",
		"uri": "/",
		"headers": {
			"User-Agent": ["curl/7.82.0"],
			"Accept": ["*/*"],
			"Accept-Encoding": ["gzip, deflate, br"],
		},
		"tls": {
			"resumed": false,
			"version": 772,
			"cipher_suite": 4865,
			"proto": "h2",
			"server_name": "example.com"
		}
	},
	"bytes_read": 0,
	"user_id": "",
	"duration": 0.000929675,
	"size": 10900,
	"status": 200,
	"resp_headers": {
		"Server": ["Caddy"],
		"Content-Encoding": ["gzip"],
		"Content-Type": ["text/html; charset=utf-8"],
		"Vary": ["Accept-Encoding"]
	}
}

This PR fixes the README and the definition of commonLogFormat.

[mohammed90]

Ha, I'm surprised nobody has noticed for 4 years. Thanks!