docs boards native_sim: Improve FUSE FS access description

The FUSE driver allows accessing a possible filesystem, independently
of the underlaying device, so let's correct the description so we do
not mix "flash" with "filesystem".

Also, one does not need to create the 'flash' folder these days. The
FUSE library will if needed.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>