Trust Tokens: Correct a typo in a comment

This change adds a space where one was missing.

Bug: None
Change-Id: I310ac3ec640a3d289db5a540db2959ede646bf2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638371
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Steven Valdez <svaldez@chromium.org>
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845140}

services/network/public/cpp/trust_token_http_headers.h

@@ -40,7 +40,7 @@ constexpr char kTrustTokensRequestHeaderSecTime[] = "Sec-Time";
 // collection of headers; and, optionally, the request's body).
 constexpr char kTrustTokensRequestHeaderSecSignature[] = "Sec-Signature";
 
-// As a request header, provides aRedemption Record obtained from a prior
+// As a request header, provides a Redemption Record obtained from a prior
 // issuance-and-redemption flow.
 constexpr char kTrustTokensRequestHeaderSecRedemptionRecord[] =
     "Sec-Redemption-Record";