btrfs-progs: Doc: Fix asciidoc grammar of btrfs-rescue

btrfs-progs: Doc: Fix asciidoc grammar of btrfs-rescue

Code block of kernel backtrace lacks leading change line, causing the
following man page result:
------
           One can determine whether zero-log is needed according to the
           kernel backtrace:

           ? replay_one_dir_item+0xb5/0xb5 [btrfs]
           ? walk_log_tree+0x9c/0x19d [btrfs]
           ? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs]
           ? btrfs_recover_log_trees+0x195/0x29c [btrfs]
           ? replay_one_dir_item+0xb5/0xb5 [btrfs]
           ? btree_read_extent_buffer_pages+0x76/0xbc [btrfs]
           ? open_ctree+0xff6/0x132c [btrfs]

       + If the errors are like above, then zero-log should be used to clear
       the log and the filesystem may be mounted normally again. The keywords
------

Not only "+" is rendered as is, but also wrong indent.

Fix it by adding change line before code block.

Signed-off-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>