Changes ACK_RTT to sRTT in CDF plot.

plot_transmission_timeline.py

     cc_algo = cc_algo.upper()
     transmission_direction = transmission_df["direction"].iloc[0]
 
-    transmission_df = transmission_df.filter(["goodput", "datetime", "ack_rtt", "goodput_rolling"])
+    transmission_df = transmission_df.filter(["goodput", "datetime", "srtt", "goodput_rolling"])
 
     # read serial csv
     serial_df = pd.read_csv(args.serial_file)