Quickstart fails with unexpected <EOF> wth graphql schema

Even after generating the supergraph.graphql schema, still getting the above error.

(It's unfortunate there is not any feedback loops on the quickstarts.)

```
const { ApolloServer } = require("apollo-server");
const { ApolloGateway } = require("@apollo/gateway");
const { readFileSync } = require("fs");

const supergraphSdl = readFileSync("./supergraph.graphql").toString();

const gateway = new ApolloGateway({
supergraphSdl,
});

const server = new ApolloServer({
gateway,
});

server
.listen()
.then(({ url }) => {
console.log(`🚀 Gateway ready at ${url}`);
})
.catch((err) => {
console.error(err);
});

Reproduce the quickstart using the shown ApolloGateway entry point and the generated supergraph.graphql file, then inspect where the unexpected EOF is reported. The work is done when the documented quickstart starts successfully with the generated schema and the failure or required setup is clearly addressed.

Written by the indexing model from the issue text.