fixing erroneous instructions in readme, should be applied to --help menu, as well

To use multiples of the same voices, voices must be in quotes: e.g "Kathy*5" vs Kathy*5. When using multiple voices the quotes go around both, e.g: "Fred*2,Kathy*2."  I updated the readme to reflect this, though you'll probably want to change the --help menu, as well.