fix translate

[Junhong-Kim]

원문과 번역본 내용이 다른 부분을 수정했습니다.

Accessing an element outside the set of known indices fails with an error:

x[3] = "world"; // Error, Property '3' does not exist on type '[string, number]'.

console.log(x[5].toString()); // Error, Property '5' does not exist on type '[string, number]'.