examples: Use DataRate for data rate parameters in wifi examples

examples/wireless/wifi-sleep.cc

@@ -109,7 +109,7 @@ PhyStateTrace(std::string context, Time start, Time duration, WifiPhyState state
 int
 main(int argc, char* argv[])
 {
-    std::string dataRate{"1Mbps"};
+    DataRate dataRate{"1Mb/s"};
     uint32_t packetSize{1000}; // bytes
     Time duration{"10s"};
     double initialEnergy{7.5}; // joule

examples/wireless/wifi-tcp.cc

@@ -73,7 +73,7 @@ int
 main(int argc, char* argv[])
 {
     uint32_t payloadSize{1472};           /* Transport layer payload size in bytes. */
-    std::string dataRate{"100Mbps"};      /* Application layer datarate. */
+    DataRate dataRate{"100Mb/s"};         /* Application layer datarate. */
     std::string tcpVariant{"TcpNewReno"}; /* TCP variant type. */
     std::string phyRate{"HtMcs7"};        /* Physical layer bitrate. */
     Time simulationTime{"10s"};           /* Simulation time. */