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>

doc/hardware/porting/board_porting.rst

@@ -536,8 +536,8 @@ files for a board named ``plank``:
      config BOARD_PLANK
              select SOC_SOC1
 
-  The Kconfig symbols :kconfig:option:`BOARD_<board>` and
-  :kconfig:option:`BOARD_<normalized_board_target>` are constructed by the build
+  The Kconfig symbols :samp:`BOARD_{board}` and
+  :samp:`BOARD_{normalized_board_target}` are constructed by the build
   system, therefore no type shall be defined in above code snippet.
 
 :file:`Kconfig`