Fix "Modification of non-creatable array value attempted, subscript -1" error and typos in README

When the base directory of backups is empty xtrabackup-helper return the following error : 
> Modification of non-creatable array value attempted, subscript -1 at /usr/local/inikup/xtrabackup-helper.pl line 300.

This patch fix that and also some typos in README file.