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>

fs/btrfs/disk-io.c

@@ -2452,7 +2452,7 @@ int btrfs_validate_super(const struct btrfs_fs_info *fs_info,
 	    (!btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID) ||
 	     !btrfs_fs_incompat(fs_info, NO_HOLES))) {
 		btrfs_err(fs_info,
-		"block-group-tree feature requires fres-space-tree and no-holes");
+		"block-group-tree feature requires free-space-tree and no-holes");
 		ret = -EINVAL;
 	}