mariabackup --help output still referst to innobackupex

The --help output of mariabackup still refers to the XtraBackup innobackupex tool in several places. As far as I can tell all these references were originally just "this tool" references, so we should replace them with references to mariabackup

I know that we also have a mariabackup --innobackupex backwards compatibility mode, but none of the options referencing innobackupex in their help output seem to be specific to that mode, they all seem to be general options.