Update part four of tutorial to be consistent. (#3329)
Update part four of tutorial to be consistent. (#3329)

The previous tutorials used `export default typography;`, so do the same in part four.
danielfarrell/gatsby,Khaledgarbaya/gatsby,fabrictech/gatsby,fabrictech/gatsby,Khaledgarbaya/gatsby,Khaledgarbaya/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,danielfarrell/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,danielfarrell/gatsby,fk/gatsby,fabrictech/gatsby,fk/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby