docs: Fix typo in qemu-nbd -P replacement

docs: Fix typo in qemu-nbd -P replacement

The suggested replacement for the deprecated 'qemu-nbd -P' refers to
'file.backing.opt' instead of 'file.file.opt'; using the example
verbatim results in:

qemu-nbd: Failed to blk_new_open 'driver=raw,offset=1m,size=100m,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file4': A block device must be specified for "file"

Correct this text, prior to actually finishing the deprecation process.

Fixes: 0ae2d54645eb
Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200123164650.1741798-2-eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>