doc: correct typo in ticket reference

The OS#4993 has nothing to do with AF_PACKET/ENOBUFS,
the proper ticket is OS#4995 as referenced later in the same file.

Change-Id: Icf13b351dc74508fc312c535d68b13b7ce9b7e1e
diff --git a/src/gb/gprs_ns2_fr.c b/src/gb/gprs_ns2_fr.c
index 4e848be..f85b1e4 100644
--- a/src/gb/gprs_ns2_fr.c
+++ b/src/gb/gprs_ns2_fr.c
@@ -96,7 +96,7 @@
 	struct osmo_fr_link *link;
 	int ifindex;
 	bool if_running;
-	/* backlog queue for AF_PACKET / ENOBUFS handling (see OS#4993) */
+	/* backlog queue for AF_PACKET / ENOBUFS handling (see OS#4995) */
 	struct {
 		/* file-descriptor for AF_PACKET socket */
 		struct osmo_fd ofd;