path for backend corrected

Frontend/main.py

@@ -12,7 +12,7 @@ import os, time, sys
 The following are the import for the backend. Just write the name of the script without .py.
 Then you can immediately use the function from the script directly in the app.
 """
-sys.path.append(os.path.join(os.path.dirname(sys.path[0]),'Backend'))
+sys.path.append(os.path.join(os.path.dirname(sys.path[0]),'backend'))
 import dqn, petrinet
 import eventlog as elog