doc: no X-Real-IP in http proxy

[fakeboboliu]

Since there's no X-Real-IP header set for the HTTP proxy, the doc seems wrong.

frp/pkg/util/vhost/reverseproxy.go

Lines 281 to 293 in ff7b8b0

	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
	// If we aren't the first proxy retain prior
	// X-Forwarded-For information as a comma+space
	// separated list and fold multiple headers into one.
	prior, ok := outreq.Header["X-Forwarded-For"]
	omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
	if len(prior) > 0 {
	clientIP = strings.Join(prior, ", ") + ", " + clientIP
	}
	if !omit {
	outreq.Header.Set("X-Forwarded-For", clientIP)
	}
	}