doc: porting_guide: fix ReStructuredText in Kconfig.plank

Using <  > in the kconfig:option role was causing the <_board> section
to be omitted from the rendered docs, and using the role was useless
anyway as the option name was not a real one.
Use the fancy :samp: role instead to achieve the same goal.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>