poke: exit with status 3 on timeout

Change-Id: Idb021fe1fa1c3d40dbcfc87d4b1dadbea6cb28f6
diff --git a/manpages/ndnpoke.rst b/manpages/ndnpoke.rst
index d92ba43..f5ae345 100644
--- a/manpages/ndnpoke.rst
+++ b/manpages/ndnpoke.rst
@@ -50,6 +50,8 @@
 
 2: Malformed command line
 
+3: No Interests received before the timeout
+
 5: Prefix registration failed
 
 Example