Wrong: All members must conform to the string index signature?

[origin](https://basarat.gitbook.io/typescript/type-system/index-signatures#all-members-must-conform-to-the-string-index-signature)

i copied the Foo and Bar into ts playground and it works.

```typescript
interface Foo {x:number; y:string}
const index: 'x'|'y' = fromSomewhere;
const foo1: Foo = { x:1, y:"a b c"}
const bar: string | number = foo1[index];
```

Read the linked index-signatures section and reproduce the Foo/Bar snippet in the TypeScript playground. Compare the playground result with the section's claim, then update the relevant documentation so the example and explanation agree; done means the documented behavior matches the verified result.

Written by the indexing model from the issue text.