fix typo 's/dowlink/downlink/g'

Change-Id: Iae66aff9eed3856f09e58116ee26ec061733b076
diff --git a/doc/tbf.txt b/doc/tbf.txt
index 6abcb0b..caddf63 100644
--- a/doc/tbf.txt
+++ b/doc/tbf.txt
@@ -52,7 +52,7 @@
       Attach PDU to LLC Frame of TBF.
       Put TBF back into FLOW state.
       Done.
-  If dowlink TBF does not exists for given TLLI, or in RELEASING state:
+  If downlink TBF does not exists for given TLLI, or in RELEASING state:
     Create new downlink TBF.
     Attach PDU to LLC Frame of TBF.
     If uplink TBF exists for given TLLI, but not in RELEASING state: