erofs: fix broken illustration in documentation

Illustration was broken after ReST conversion by accident.
(checked by 'make SPHINXDIRS="filesystems" htmldocs')

Link: https://lore.kernel.org/r/20210510162506.28637-1-xiang@kernel.org
Fixes: e66d8631

 ("docs: filesystems: convert erofs.txt to ReST")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>