Bluetooth: IPSP: Fix pointing to invalid file

sample/net/README no longer exists so this removes its mention since
it is already described how to use echo-client at the end of the
document. Also state that CONFIG_BT_6LOWPAN shall be set in order to
use Bluetooth 6LoWPAN module with Linux.

Fixes #9727

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>