doc: net: Fix the path and description of the IP stack

As the Contiki uIP stack is no more there remove any reference
to it. The path to IP stack is also set correctly.

Change-Id: Ia2e10514f173cf8083d6dd258111a901987bd038
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>