btrfs-progs: docs: dump-tree, fix formatting of --dfs/--bfs

Description that's not one paragraph needs to be joned to the following
section with +.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-inspect-internal.asciidoc

@@ -94,6 +94,7 @@ use with '-b', print all children tree blocks of '<block_num>'
 +
 use depth-first search to print trees, the nodes and leaves are
 intermixed in the output
++
 --bfs::::
 (default since 5.3)
 +