Update sigstore_rekor.proto

Remove timestamp from checkpoint docs.

Signed-off-by: Appu <[email protected]>

protos/sigstore_rekor.proto

@@ -34,9 +34,9 @@ message KindVersion {
         string version = 2 [(google.api.field_behavior) = REQUIRED];
 }
 
-// The checkpoint contains a signature of the tree head (root hash),
+// The checkpoint must contain a signature of the tree head (root hash),
 // size of the tree, the transparency log's unique identifier (log ID),
-// hostname and the current time.
+// and hostname. It may also be following by any optional data.
 // The result is a string, the format is described here
 // https://github.com/transparency-dev/formats/blob/main/log/README.md
 // The details are here https://github.com/sigstore/rekor/blob/a6e58f72b6b18cc06cefe61808efd562b9726330/pkg/util/signed_note.go#L114