tour: Incorrect calculation of days until Saturday

[Nikilimanjaro]

Context: https://go.dev/tour/flowcontrol/10

Description: The code incorrectly calculates the number of days until Saturday by directly comparing time.Saturday with today + n. This causes errors when today + n exceeds 6 (the valid range for days of the week).

Steps to Reproduce:

1. Run the provided code.
2. The program may output incorrect results depending on the current day.

Expected Behavior: The program should correctly output:

• "Today." if today is Saturday.
• "Tomorrow." if tomorrow is Saturday.
• "In two days." if Saturday is in two days.
• "Too far away." if Saturday is more than two days away.

Actual Behavior: The code may produce incorrect outputs when today + n exceeds 6.

Solution: Replace the comparison logic with:

daysUntilSaturday := (time.Saturday - today + 7) % 7