message: Use Vega 4 in readme
diff: ## Basic Example
  -You can import Vega-Embed from a local copy or (as shown below) [from jsDelivr](hhttps://www.jsdelivr.com/package/npm/vega-embed). Please replace `[VERSION]` with the correct [Vega](https://www.jsdelivr.com/package/npm/vega), [Vega-Lite](https://www.jsdelivr.com/package/npm/vega-lite), and [Vega-Embed](https://www.jsdelivr.com/package/npm/vega-embed) versions. We recommend that you specify the major versions (`vega@3`, `vega-lite@2`, `vega-embed@3`).
  +You can import Vega-Embed from a local copy or (as shown below) [from jsDelivr](hhttps://www.jsdelivr.com/package/npm/vega-embed). Please replace `[VERSION]` with the correct [Vega](https://www.jsdelivr.com/package/npm/vega), [Vega-Lite](https://www.jsdelivr.com/package/npm/vega-lite), and [Vega-Embed](https://www.jsdelivr.com/package/npm/vega-embed) versions. We recommend that you specify the major versions (`vega@4`, `vega-lite@2`, `vega-embed@3`).
  ```html
  <!DOCTYPE html>
  <html>
  <head>
  - <!-- Import Vega 3 & Vega-Lite 2 (does not have to be from CDN) -->
  + <!-- Import Vega 4 & Vega-Lite 2 (does not have to be from CDN) -->
  <script src="https://cdn.jsdelivr.net/npm/vega@[VERSION]"></script>
  <script src="https://cdn.jsdelivr.net/npm/vega-lite@[VERSION]"></script>
  <!-- Import vega-embed -->
  