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

  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

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

  as shown above.

  as shown above.