graphql-playground-react@1.7.8 not found on cdn.jsdelivr.net

Hey guys, thanks for the handy tutorial.

I've found an error running the `Graphql Playground`. The `apollo-server-core@2.2.0-alpha.2` (`apollo-server@2.2.0-alpha.2` dep) is setting the `@apollographql/graphql-playground-react@1.7.8` version to `1.7.8`, which doesn't exist anymore on the `cdn.jsdelivr.net`. Here's the playground html source:

```html

```

Updating `apollo-server` to it's current version `2.5.0` fixes the problem, as it changes the `apollographql/graphql-playground-react` version to `1.7.10`.

I will send you guys a PR to update both server/client dependencies, if its ok.

检查教程的服务器和客户端依赖清单，找出生成 GraphQL Playground HTML 的 Apollo 软件包版本。验证依赖更新后所引用的 jsDelivr 资源能够解析，然后运行教程现有的检查或启动流程，以确认 playground 能够加载。

由索引模型根据 Issue 内容生成。