Fix incorrect spec reference

RAI is defined in 3GPP TS 24.008 ยง 10.5.5.15

Change-Id: I484485d8c4c56b58dfecc1193bcdd48e61957422
diff --git a/include/osmocom/gsm/protocol/gsm_04_08.h b/include/osmocom/gsm/protocol/gsm_04_08.h
index 01ab599..93b8c59 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08.h
@@ -1648,7 +1648,7 @@
 #define GSM48_MID_TMSI_LEN	(GSM48_TMSI_LEN + 2)
 #define GSM48_MI_SIZE 32
 
-/* Chapter 10.4.4.15 */
+/* 3GPP TS 24.008 § 10.5.5.15 Routing area identification */
 struct gsm48_ra_id {
 	uint8_t digits[3];	/* MCC + MNC BCD digits */
 	uint16_t lac;		/* Location Area Code */