fix: use slice instead of substr(deprecated)

[youngjae99]

Resolves https://github.com/splitbee/notion-api-worker/issues/72

[tobiaslins]

unfortunately slice can't be used like substr but I just fixed the fn now.

export const idToUuid = (path: string): string =>
  `${path.slice(0, 8)}-${path.slice(8, 12)}-${path.slice(12, 16)}-${path.slice(16, 20)}-${path.slice(20)}`;