Trust Tokens: Correct a comment describing the batch size as optional

It was changed around a month ago to be mandatory.



Change-Id: I4c76887f77aee2dc62efb4c7ce034541cc96e9bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264840
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782000}

services/network/trust_tokens/trust_token_key_commitment_parser.cc

@@ -144,7 +144,7 @@ const char kTrustTokenKeyCommitmentKeyField[] = "Y";
 
 // https://docs.google.com/document/d/1TNnya6B8pyomDK2F1R9CL3dY10OAmqWlnCxsWyOBDVQ/edit#bookmark=id.6wh9crbxdizi
 // {
-//   "batchsize" : ..., // Optional batch size; value of type int.
+//   "batchsize" : ..., // Batch size; value of type int.
 //   "srrkey" : ...,    // Required Signed Redemption Record (SRR)
 //                      // verification key, in base64.
 //