tbf_dl.cpp: Fix typo in log line

Change-Id: I9fdea4246c95897f3e72604981597db828a219a3
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 4efb5a4..e3c1ff4 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -846,7 +846,7 @@
 			need_poll) {
 		if (m_dl_ack_requested) {
 			LOGPTBFDL(this, LOGL_DEBUG,
-				  "Scheduling Ack/Nack polling, because is was requested explicitly "
+				  "Scheduling Ack/Nack polling, because it was requested explicitly "
 				  "(e.g. first final block sent).\n");
 		} else if (need_poll) {
 			LOGPTBFDL(this, LOGL_DEBUG,