shorten comment and add links to relevant sources

`%x` - https://go.dev/play/p/Fylce70N2Zl
Leading Zeros - mozilla/tls-observatory#245

Signed-off-by: Rhys Evans <rhys.rr@googlemail.com>

prober/tls.go

@@ -72,9 +72,8 @@ func getLastChainExpiry(state *tls.ConnectionState) time.Time {
 
 func getSerialNumber(state *tls.ConnectionState) string {
 	cert := state.PeerCertificates[0]
-	// Actual serial number = 0B:FF:BC:11:F1:90:7D:02:AF:71:9A:FC:D6:4F:B2:53
-	// serialNumber := cert.SerialNumber.Text(16) // drops leading zeros outputs = BFFBC11F1907D02AF719AFCD64FB253 in lower case, telgraf follows this https://github.com/influxdata/telegraf/blob/a9c91f162ddbe453364f68a89799535c43328a3c/plugins/inputs/x509_cert/x509_cert.go#L218
-	// https://github.com/atc0005/check-cert retains the leading zero with some aditional formatting
+	// Using `cert.SerialNumber.Text(16)` will drop the leading zeros when converting the SerialNumber to String, see https://github.com/mozilla/tls-observatory/pull/245.
+	// To avoid that, we format in lowercase the bytes with `%x` to base 16, with lower-case letters for a-f, see https://go.dev/play/p/Fylce70N2Zl.
 
 	return fmt.Sprintf("%x", cert.SerialNumber.Bytes())
 }