doc: Update "Linking Zephyr Within a Partition" section

The method used to link code partition, as defined by
zephyr,code-partition has been modified in Zephyr 1.14. Update the
"Linking Zephyr Within a Partition" section to reflect the change.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>