Added .next folder to gitignore for examples/blog-starter (#14003)
Added .next folder to gitignore for examples/blog-starter (#14003)

```.next``` folder wasn't in the gitignore file causing .next folder to be tracked by git
zeit/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,flybayer/next.js