Add missing `arrow` in documentation

[gillstrom]

If I'm not mistaken arrows is missing in those functions.

[anthonyshort]

Whoops! Thanks.

[stevenmiller888]

You don't need an arrow here

e.g. https://github.com/lukehoban/es6features#enhanced-object-literals

[anthonyshort]

@stevenmiller888 Only if I didn't also have a : there.

render ({ children }) {
  return <button class="my-button">{children}</button>
}

vs

render: ({ children }) => {
  return <button class="my-button">{children}</button>
}

[stevenmiller888]

Whoops you're right

[anthonyshort]

Pretty much why I don't use those fancy functions in object literals. It's too confusing with the 10 different ways of declaring functions.