spanner: fix timestamp bounds docs

Change-Id: I53f29ae79672ccf0241a4c03fa6df6987aa6c7b7
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/53730
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Shanika Kuruppu <skuruppu@google.com>
diff --git a/spanner/timestampbound.go b/spanner/timestampbound.go
index 68a930d..fd017c4 100644
--- a/spanner/timestampbound.go
+++ b/spanner/timestampbound.go
@@ -117,12 +117,12 @@
 //
 // 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