Combines this year-month with a day-of-month to create a {@code LocalDate}.
This returns a {@code LocalDate} formed from this year-month and the specified day-of-month.
The day-of-month value must be valid for the year-month.
This method can be used as part of a chain to produce a date:
LocalDate date = year.atMonth(month).atDay(day);
@param dayOfMonth the day-of-month to use, from 1 to 31
@return the date formed from this year-month and the specified day, not null
@throws DateTimeException if the day is invalid for the year-month
@see #isValidDay(int)