add note about $ in command line blocks

_episodes/04-argparse.md

@@ -63,8 +63,8 @@ $ python argv_example1.py one 2 3.0 four,five
 {: .language-bash }
 ~~~
 START
-argv_example1.py one 2 3.0 four,five
-argv_example1.py
+['code/argv_example1.py', 'one', '2', '3.0', 'four,five']
+code/argv_example1.py
 one
 2
 3.0
@@ -73,6 +73,12 @@ END
 ~~~
 {: .output }
 
+**Note about Bash code blocks:**
+we include `$` to represent the shell prompt in commands like those above
+(`cat argv_example.py` & `python argv_example1.py one 2 3.0 four,five`).
+When reproducing these commands on your own system,
+you should leave that extra `$` symbol out.
+
 > ## 4.1. Carry the Zero
 >
 > - What does `sys.argv[0]` return when you run it in a script?