message: made example programmer friendlier
  it cost me an hour to fix this in my simple test. ;)
diff: @@ -49,7 +49,7 @@ const elements = [
  { id: 'e2-3', source: '2', target: '3' },
  ];
  -export default () => <ReactFlow elements={elements} />;
  +export default () => <div style={{ height: 300 }}><ReactFlow elements={elements} /></div>;
  ```
  import Flow from './index';
  @@ -86,11 +86,14 @@ export default () => {
  const onConnect = (params) => setElements((els) => addEdge(params, els));
  return (
  + <div style={{ height: 300 }}>
  <ReactFlow
  elements={elements}
  onElementsRemove={onElementsRemove}
  onConnect={onConnect}
  + deleteKeyCode={46} /* 'delete'-key */
  />
  + </div>
  );
  }
  ```
  