fuse: fix references to fuse.rst -> fuse/fuse.rst

Commit 6be0ddb2 ("Documentation: fuse: Consolidate FUSE docs into its
own subdirectory") moved fuse docs to a subdirectory but didn't update
references inside the kernel tree.

Fixes: 6be0ddb2 ("Documentation: fuse: Consolidate FUSE docs into its own subdirectory")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508261621.EaNMWVjm-lkp@intel.com/


Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>