"[T]he lawsuit of the Georgia plaintiff, Mary Doe, is also justiciable and is not mooted by the fact that her pregnancy has terminated. The physicians who joined her lawsuit are not currently being prosecuted or threatened with prosecution under Georgia law, but they have alleged a credible fear of future prosecution if they participate in abortions. This is sufficient to give them standing. See Epperson v. Arkansas, 393 U.S. 97 (1968) (recognizing right of school teaher who had not yet been charged criminally to challenge state anti-evolution statute)."