Format plot_congestion.py with black

Signed-off-by: Mithilesh Lohar <128236295+mithlohar144@users.noreply.github.com>

Author: mithlohar144

flow/util/plot_congestion.py

@@ -4,6 +4,7 @@
 import re
 import os
 
+
 def usage(script):
     print(
         f"Usage: {script} <sweep_name> <output_png> <congestion_file...> <sweep_value...>",