TTS rate velocity is wrong in the doc

[jackie-d]

In the doc rate parameter is signed as 0 ~ 1, but in implementation I can see no difference between 0.9 and 0.4, but instead we can make play different rate with i.e. 0.75 and 1.5.

Please correct!

Thanks and great job btw!

[gormlabenz]

I think it is between 1 and 2