I work for a company where recently I was issued my first verbal warning for tardiness.I have worked here for 7 years.My supervisor showed my private document to another employee who gossips a lot.I find her lack of discretion very unprofessional.Does this violate the privacy act?If so what should I do about it?

Unless you work for a federal agency then no, although it is very unprofessional.

What "privacy act"? There is no one privacy act. ANY question on law needs to include your general locale as laws vary by area. Or we need to know the specific act you refer to.

But this does not appear to be illegal, though it is unprofessional.