modify comment
diff --git a/tomb.go b/tomb.go
index 0f4971e..0c0445c 100644
--- a/tomb.go
+++ b/tomb.go
@@ -41,7 +41,7 @@
// and the reason for its death.
//
// The zero value of a Tomb assumes that a goroutine is about to be
-// created or already alive. Once Kill is called with an
+// created or already alive. Once Kill or Killf is called with an
// argument that informs the reason for death, the goroutine is in
// a dying state and is expected to terminate soon. Right before the
// goroutine function or method returns, Done must be called to inform
