btrfs: fix typo in error message in btrfs_validate_super()

There's a typo in an error message when checking the block group tree
feature, it mentions fres-space-tree instead of free-space-tree. Fix
that.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>