Update tuple docs to match post-2.7 behaviour
Update tuple docs to match post-2.7 behaviour

Tuples were changed in 2.7 (see https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html , section "Fixed Length Tuples") to not allow assignments outside of the declared indexes. This commit updates the handbook docs to match this behaviour.
Microsoft/TypeScript-Handbook