[3173] Fixed the doxygen error in perfdhcp.

tests/tools/perfdhcp/stats_mgr.h

@@ -260,6 +260,7 @@ public:
         /// assumed dropped. Negative value disables it.
         /// \param archive_enabled if true packets archive mode is enabled.
         /// In this mode all packets are stored throughout the test execution.
+        /// \param boot_time Holds the timestamp when perfdhcp has been started.
         ExchangeStats(const ExchangeType xchg_type,
                       const double drop_time,
                       const bool archive_enabled,