Update protos/sigstore_rekor.proto

Co-authored-by: Hayden B <[email protected]>
Signed-off-by: Appu <[email protected]>

protos/sigstore_rekor.proto

@@ -34,7 +34,7 @@ message KindVersion {
         string version = 2 [(google.api.field_behavior) = REQUIRED];
 }
 
-// The checkpoint must contain a signature of the tree head (root hash),
+// The checkpoint MUST contain a signature of the tree head (root hash),
 // size of the tree and the transparency log's unique identifier (log ID).
 // It MAY also be followed 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