time: fix 400 years offset in comment

Fixes #21043

Change-Id: I12db4f67ddee80500ff323f84e8fa34511650e63
Reviewed-on: https://go-review.googlesource.com/49230Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/time.go

@@ -370,7 +370,7 @@ func (d Weekday) String() string { return days[d] }
 // everywhere.
 //
 // The calendar runs on an exact 400 year cycle: a 400-year calendar
-// printed for 1970-2469 will apply as well to 2370-2769. Even the days
+// printed for 1970-2369 will apply as well to 2370-2769. Even the days
 // of the week match up. It simplifies the computations to choose the
 // cycle boundaries so that the exceptional years are always delayed as
 // long as possible. That means choosing a year equal to 1 mod 400, so