Bluetooth: IPSP: Fix pointing to invalid file
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 <8530c5ea66a1bdbc08f98bfcf183c8be901b5990@intel.com>

zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,kraj/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr