Update README: Azkaban2 -> Azkaban 3
Update README: Azkaban2 -> Azkaban 3

Just to avoid confusion. The current master is version 3.0.0, so making the README reflect that.
jackrjli/azkaban,poporisil/azkaban3,evlstyle/azkaban,sunghyuk/azkaban,mradamlacey/azkaban,chengren311/azkaban,reallocf/azkaban,azkaban/azkaban,mariacioffi/azkaban,HappyRay/azkaban,reallocf/azkaban,jackrjli/azkaban,binhnv/azkaban,erwa/azkaban,inoviaazkaban/azkaban,researchgate/azkaban,poporisil/azkaban3,mradamlacey/azkaban,mariacioffi/azkaban,chengren311/azkaban,erwa/azkaban,evlstyle/azkaban,sunghyuk/azkaban,binhnv/azkaban,erwa/azkaban,logiclord/azkaban,HappyRay/azkaban,researchgate/azkaban,binhnv/azkaban,logiclord/azkaban,binhnv/azkaban,HappyRay/azkaban,mariacioffi/azkaban,azkaban/azkaban,mradamlacey/azkaban,erwa/azkaban,inoviaazkaban/azkaban,reallocf/azkaban,mradamlacey/azkaban,sunghyuk/azkaban,poporisil/azkaban3,jackrjli/azkaban,erwa/azkaban,reallocf/azkaban,HappyRay/azkaban,azkaban/azkaban,poporisil/azkaban3,binhnv/azkaban,jackrjli/azkaban,evlstyle/azkaban,johnyu0520/azkaban,chengren311/azkaban,azkaban/azkaban,researchgate/azkaban,evlstyle/azkaban,poporisil/azkaban3,HappyRay/azkaban,researchgate/azkaban,inoviaazkaban/azkaban,azkaban/azkaban,mariacioffi/azkaban,johnyu0520/azkaban,chengren311/azkaban,johnyu0520/azkaban,sunghyuk/azkaban,logiclord/azkaban,reallocf/azkaban,jackrjli/azkaban,azkaban/azkaban,johnyu0520/azkaban,mradamlacey/azkaban,sunghyuk/azkaban,logiclord/azkaban,chengren311/azkaban,HappyRay/azkaban,mariacioffi/azkaban