net: doc: Fix incorrect netcat note for qemu setup

If netcat is used with UDP, then one cannot press CTRL-c
as netcat returns immediately to the caller. For TCP the
CTRL-c is needed so move the note to TCP section of this
document.

Change-Id: I936a89e7a7ce8318602c3deae8513007a4620c80
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>