time: use secondsPerMinute instead of 60

It's maybe better.

Change-Id: I7929e93a95c96676915bc24f2f7cce4e73b08c59
GitHub-Last-Rev: a8c2bb6cafe78090f35c3b194e270e301255be89
GitHub-Pull-Request: golang/go#26685
Reviewed-on: https://go-review.googlesource.com/126623Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/time.go

@@ -933,7 +933,7 @@ func (t Time) AddDate(years int, months int, days int) Time {
 
 const (
 	secondsPerMinute = 60
-	secondsPerHour   = 60 * 60
+	secondsPerHour   = 60 * secondsPerMinute
 	secondsPerDay    = 24 * secondsPerHour
 	secondsPerWeek   = 7 * secondsPerDay
 	daysPer400Years  = 365*400 + 97