iosys-map: fix typo

 * the same driver for allocation, read and write operations.

 *

 * Open-coding access to :c:type:`struct iosys_map <iosys_map>` is considered

 * bad style. Rather then accessing its fields directly, use one of the provided

 * bad style. Rather than accessing its fields directly, use one of the provided

 * helper functions, or implement your own. For example, instances of

 * :c:type:`struct iosys_map <iosys_map>` can be initialized statically with

 * IOSYS_MAP_INIT_VADDR(), or at runtime with iosys_map_set_vaddr(). These