Documentation - Correction required in Swapping to expressMiddleware
### Description

After trying to implement the [Swapping to expressMiddleware](https://www.apollographql.com/docs/apollo-server/api/standalone/#swapping-to-expressmiddleware) for healthchecks I noticed an error in the documentation, ended up costing me an hour or two.

The documentation is [incorrect](https://github.com/apollographql/apollo-server/blob/1b940cf682ade7bb699560e6cb316372f2814275/docs/source/api/standalone.mdx#L197) at `standalone.mdx` (I would create a PR but I don't have write access to the repository, I'd prefer to just create the PR myself)

```
app.use('/',
  cors<cors.CorsRequest>(),
  // 50mb is the limit that `startStandaloneServer` uses, but you may configure this to suit your needs
  bodyParser.json({ limit: '50mb' }),
  // expressMiddleware accepts the same arguments:
  // an Apollo Server instance and optional configuration options
  expressMiddleware(server, {
    context: async ({ req }) => ({ token: req.headers.token }),
  }),
);
```

should be changed to 

```
app.use('/graphql',
  cors<cors.CorsRequest>(),
  // 50mb is the limit that `startStandaloneServer` uses, but you may configure this to suit your needs
  bodyParser.json({ limit: '50mb' }),
  // expressMiddleware accepts the same arguments:
  // an Apollo Server instance and optional configuration options
  expressMiddleware(server, {
    context: async ({ req }) => ({ token: req.headers.token }),
  }),
);
```
