drm/gem-shmem: Fix typos in documentation

drm/gem-shmem: Fix typos in documentation

Fix the compile-time warnings

Warning: drm_gem_shmem_helper.c:104 function parameter 'shmem' not described in 'drm_gem_shmem_init'
Warning: drm_gem_shmem_helper.c:104 function parameter 'size' not described in 'drm_gem_shmem_init'

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Fixes: e3f4bdaf2c5b ("drm/gem/shmem: Extract drm_gem_shmem_init() from drm_gem_shmem_create()")
Link: https://patch.msgid.link/20251209140141.94407-2-tzimmermann@suse.de