server: fix duplicate 'is' typo in comment (#12985)

server/download.go

@@ -465,7 +465,7 @@ type downloadOpts struct {
 // downloadBlob downloads a blob from the registry and stores it in the blobs directory
 func downloadBlob(ctx context.Context, opts downloadOpts) (cacheHit bool, _ error) {
 	if opts.digest == "" {
-		return false, fmt.Errorf(("%s: %s"), opts.mp.GetNamespaceRepository(), "digest is is empty")
+		return false, fmt.Errorf(("%s: %s"), opts.mp.GetNamespaceRepository(), "digest is empty")
 	}
 
 	fp, err := GetBlobsPath(opts.digest)