iosys-map: Fix typo in documentation

It's one argument, vaddr_iomem, not 2 (vaddr and _iomem).

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220610232130.2865479-3-lucas.demarchi@intel.com