doc: fix FIXED_PARITION_ID typo

Numeric flash area ID is obtained by passing DTS node label to

:c:macro:`FIXED_PARTITION_ID()`; for example to obtain ID number

for ``slot0_partition``, user would invoke ``FIXED_PARITION_ID(slot0_partition)``.

for ``slot0_partition``, user would invoke ``FIXED_PARTITION_ID(slot0_partition)``.

All :c:macro:`FIXED_PARTITION_` macros take DTS node labels as partition

identifiers.

.. code-block:: c

   const struct flash_area *my_area;

   int err = flash_area_open(FIXED_PARITION_ID(slot0_partition), &my_area);

   int err = flash_area_open(FIXED_PARTITION_ID(slot0_partition), &my_area);

   if (err != 0) {

   	handle_the_error(err);