Fix typo in logging

DL-TBD was errorneously printed as UL-TBF.

Change-Id: I94a224c0339a062e4c8d5aa6c4c858f3f0298a0d
diff --git a/src/bts.cpp b/src/bts.cpp
index e25c47b..61c9321 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -986,7 +986,7 @@
 		if (ms)
 			LOGP(DRLCMAC, LOGL_NOTICE, "PACKET CONTROL ACK with "
 			     "unknown TBF corresponds to MS with IMSI %s, TA %d, "
-			     "uTBF (TFI=%d, state=%s), uTBF (TFI=%d, state=%s)\n",
+			     "uTBF (TFI=%d, state=%s), dTBF (TFI=%d, state=%s)\n",
 			     ms->imsi(), ms->ta(),
 			     ms->ul_tbf() ? ms->ul_tbf()->tfi() : 0,
 			     ms->ul_tbf() ? ms->ul_tbf()->state_name() : "None",