Update note in bandit20 -> bandit21

wargames/bandit/bandit21.md

@@ -12,14 +12,12 @@ the connection and compares it to the password in the previous level
 (bandit20). If the password is correct, it will transmit the
 password for the next level (bandit21).
 
- **NOTE:** To beat this level, you need to login twice: once to run
-the setuid command, and once to start a network daemon to which the
-setuid will connect.
+ **NOTE:** [Changes to the infrastructure](http://overthewire.org/help/sshinfra.html) made this level more difficult. You will need to figure out a way to launch multiple commands in the same Docker instance.
 
  **NOTE 2:** Try connecting to your own network daemon to see if it
 works as you think
 
 Commands you may need to solve this level
 -----------------------------------------
-ssh, nc, cat
+ssh, nc, cat, bash, screen, tmux