Update install_prodstack.rst

After installing Fullstack via Vagrant, the user needs to know the credentials to login to edX
I don't think I understand your question.  The Vagrant install just added an entry into my /etc/hosts file with preview.localhost pointing to 192.168.33.10.  I ran into the username/pwd block when entering preview.localhost or 192.168.33.10 into my browser.  I finally found the right credentials after a lot of Googling (edx/edx).