spanner: fix timestamp bounds docs
@@ -117,12 +117,12 @@ const (
//
// Cloud Spanner continuously garbage collects deleted and overwritten data in
// the background to reclaim storage space. This process is known as "version
-// GC". By default, version GC reclaims versions after they are four hours
-// old. Because of this, Cloud Spanner cannot perform reads at read timestamps
-// more than four hours in the past. This restriction also applies to
-// in-progress reads and/or SQL queries whose timestamps become too old while
-// executing. Reads and SQL queries with too-old read timestamps fail with the
-// error ErrorCode.FAILED_PRECONDITION.
+// GC". By default, version GC reclaims versions after they are one hour old.
+// Because of this, Cloud Spanner cannot perform reads at read timestamps more
+// than one hour in the past. This restriction also applies to in-progress reads
+// and/or SQL queries whose timestamps become too old while executing. Reads and
+// SQL queries with too-old read timestamps fail with the error
+// ErrorCode.FAILED_PRECONDITION.
type TimestampBound struct {
mode timestampBoundType
d time.Duration
