Fix location of production server file (#5731)
Fix location of production server file (#5731)

(At least with my set up, mac osx, node 10.13.0, yarn 1.12.1)

Couldn't get the server to start with `npm start`, this appears to fix that
JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,BlancheXu/test,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,BlancheXu/test,BlancheXu/test