doc: services: storage: flash_map: Fix include references

We have moved all header files under include/zephyr/ for Zephyr 3.1.
Update the document accordingly.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>

Author: yashi

doc/services/storage/flash_map/flash_map.rst

@@ -3,7 +3,7 @@
 Flash map
 #########
 
-The ``<storage/flash_map.h>`` API allows accessing information about device
+The ``<zephyr/storage/flash_map.h>`` API allows accessing information about device
 flash partitions via :c:struct:`flash_area` structures.
 
 Each :c:struct:`flash_area` describes a flash partition. The API provides access
@@ -28,7 +28,7 @@ The main examples are :c:func:`flash_area_read` and :c:func:`flash_area_write`.
 These functions are basically wrappers around the flash API with additional
 offset and size checks, to limit flash operations to a predefined area.
 
-Most ``<storage/flash_map.h>`` API functions require a :c:struct:`flash_area` object pointer
+Most ``<zephyr/storage/flash_map.h>`` API functions require a :c:struct:`flash_area` object pointer
 characterizing the flash area they will be working on. There are two possible
 methods to obtain such a pointer: