fix spelling errors detected by lintian

Change-Id: I381618eb55cc513cfa9c2e384c27cead0935c8bf
diff --git a/src/tbf.cpp b/src/tbf.cpp
index a099322..9856c7f 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -1208,7 +1208,7 @@
 
 	if (direction == GPRS_RLCMAC_DL_TBF && !is_control_ts(ts)) {
 		LOGPTBF(this, LOGL_NOTICE,
-			"Cannot poll for downlink assigment, because MS cannot reply. (TS=%d, first common TS=%d)\n",
+			"Cannot poll for downlink assignment, because MS cannot reply. (TS=%d, first common TS=%d)\n",
 			ts, first_common_ts);
 		poll_ass_dl = 0;
 	}