suggestion for the first example in Render Props section

Currently the first example we see reads:

<DataProvider render={data => (
  <h1>Hello {data.target}</h1>
)}/>

Why not extend this initial example further and show what a DataProvider component might look like?
e.g. something like...

const DataProvider = props => {
  const data = {
    target: "World"
  }
  return props.render(data);
}

Granted, this is a fairly dumb example and not something you'd use a render prop for, but a dumb example might help solidify the basic concept before leading into more useful examples and conversations. Thoughts?

開啟「Render Props」章節，並一併檢視其中的第一個範例與 issue 中提議的 DataProvider 範例。更新介紹範例，讓 provider 實作與 render prop 之間的關係更清楚；在後續範例之前就能理解基本概念時即完成。