UserModeLinux-HOWTO.txt: fix a typo

  will cause serial line 3 to accept input on the host's /dev/tty3 and

  will cause serial line 3 to accept input on the host's /dev/tty2 and

  display output on an xterm.  That's a silly example - the most common

  use of this syntax is to reattach the main console to stdin and stdout

  as shown above.