message: Fix a dead link in the upgrade guide
diff: @@ -20,7 +20,7 @@ import differenceInCalendarISOYears from 'date-fns/differenceInCalendarISOYears'
  Starting with v2 `format` and `parse` uses [Unicode tokens].
  -See [Unicode Tokens doc](./unicodeTokens.md) for more details.
  +See [Unicode Tokens doc](https://date-fns.org/docs/Unicode-Tokens) for more details.
  ### String arguments
  