f2fs: fix wrong description in document

As reported in bugzilla, default value of DEF_RAM_THRESHOLD was fixed by
commit 29710bcf ("f2fs: fix wrong percentage"), however leaving wrong
description in document, fix it.

https://bugzilla.kernel.org/show_bug.cgi?id=205203



Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>