Correct a typo in trust_token_request_signing_helper_unittest

A variable was incorrectly named: the test extracts a timestamp into
a variable currently called "retrieved_url_spec". This change fixes this
by renaming the variable.

TBR=csharrison

Change-Id: Iee866ddcc9efce6e85e81e90791f461b96ac7e23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250941Reviewed-by: David Van Cleve <davidvc@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779514}

services/network/trust_tokens/trust_token_request_signing_helper_unittest.cc

@@ -500,9 +500,9 @@ TEST_F(TrustTokenRequestSigningHelperTest, SignAndVerifyTimestampHeader) {
   std::string signature_string;
   ASSERT_NO_FATAL_FAILURE(
       AssertHasSignatureAndExtract(*my_request, &signature_string));
-  std::string retrieved_url_spec;
+  std::string retrieved_timestamp;
   ASSERT_NO_FATAL_FAILURE(AssertDecodesToCborAndExtractField(
-      signature_string, "sec-time", &retrieved_url_spec));
+      signature_string, "sec-time", &retrieved_timestamp));
 }
 
 // Test a round-trip sign-and-verify additionally signing over the destination