Change setState call to use functional form (#6401)
Change setState call to use functional form (#6401)

Not important to the function of the tutorial, as it's React specific and not Gatsby specific, but the example using state for the counter should use the functional form of `setState`. This example is so trivial that this instance would never really run up against the async nature of `setState`, so I understand if you wouldn't want to accept this PR. However, my feeling is that as this tutorial is geared towards those that might be new to React, we should teach them the "right" ways, so they don't get bit later.
0x80/gatsby,0x80/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,fk/gatsby,gatsbyjs/gatsby