message: doc: fix guide
diff: @@ -75,6 +75,7 @@ Exception are:
  - `ScreenSpaceEventHandler` > `ScreenSpaceEvent`
  - `xCollection` > `x`
  - `CustomDataSource` > `Entity`
  +- `GroundPrimitiveCollection` > `GroundPrimitive`
  e.g.: `Scene` > `Camera` or `Camera` > `Entity` is not recommended.
  @@ -182,14 +183,17 @@ const ExampleComponent = () => {
  let viewer; // Cesium.Viewer
  return (
  - <Viewer ref={e => { viewer = !!e ? e.cesiumElement : undefined; }} />
  + <Viewer
  + ref={e => {
  + viewer = !!e ? e.cesiumElement : undefined;
  + }}
  + />
  );
  -}
  +};
  ```
  ```jsx
  class ExampleComponent extends React.PureComponent {
  -
  entity = React.createRef(); // entity.current = Cesium.Entity
  render() {
  