Fixed link to introducing-graphiql (#6640)
Fixed link to introducing-graphiql (#6640)

Seems that the section is actually under `part-five`, not `part-four`
gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,0x80/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,mingaldrichgan/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby