Set dev variable consistent with examples (#939)

Author: timneutkens

README.md

@@ -376,7 +376,8 @@ const { createServer } = require('http')
 const { parse } = require('url')
 const next = require('next')
 
-const app = next({ dev: true })
+const dev = process.env.NODE_ENV !== 'production'
+const app = next({ dev })
 const handle = app.getRequestHandler()
 
 app.prepare().then(() => {