Fix small error in README (TheCodedSelf#23)

Tuesday is 2, not 3.

Author: ducky-hong

README.md

@@ -49,7 +49,7 @@ let anotherExpression = CronExpression(minute: "30", hour: "11", day: "9", month
 ```
 ```swift
 // Every tuesday at 6:00pm
-let everyTuesday = CronExpression(minute: "0", hour: "18", weekday: "3")
+let everyTuesday = CronExpression(minute: "0", hour: "18", weekday: "2")
 ```
 
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