Fix Javadoc warnings

Change-Id: Icf8b6b6f77ea80d3975aa975226a178145067adf
diff --git a/utils/misc/src/main/java/org/onlab/util/HexString.java b/utils/misc/src/main/java/org/onlab/util/HexString.java
index b04dd04..95317ab 100644
--- a/utils/misc/src/main/java/org/onlab/util/HexString.java
+++ b/utils/misc/src/main/java/org/onlab/util/HexString.java
@@ -117,6 +117,7 @@
      * If separator is the empty string or null, then no separator will be considered.
      *
      * @param values hex string to be converted
+     * @param separator regex for separator
      * @return converted byte array
      * @throws NumberFormatException if input hex string cannot be parsed
      */