Fix Javadoc warnings

Change-Id: Icf8b6b6f77ea80d3975aa975226a178145067adf
diff --git a/core/api/src/main/java/org/onosproject/net/flow/FlowEntry.java b/core/api/src/main/java/org/onosproject/net/flow/FlowEntry.java
index 1511d21..c56abbf 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/FlowEntry.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/FlowEntry.java
@@ -71,6 +71,7 @@
     /**
      * Returns the time this flow rule has been applied.
      *
+     * @param unit time unit the result will be converted to
      * @return time in the requested {@link TimeUnit}
      */
     long life(TimeUnit unit);