Signed Exchange: Fix error message

Fix error message by making it a proper sentence.

Bug: 803774
Change-Id: I30feb2f5b9aa92653793db92556aaf29371673ec
Reviewed-on: https://chromium-review.googlesource.com/c/1350962Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#611049}

content/browser/web_package/signed_exchange_envelope.cc

@@ -95,8 +95,9 @@ bool ParseRequestMap(const cbor::Value& value,
   // - headers[0][`:method`] is not `GET`. [spec text]
   if (method_str != "GET") {
     signed_exchange_utils::ReportErrorAndTraceEvent(
-        devtools_proxy, base::StringPrintf("Request method must be GET, but %s",
-                                           method_str.as_string().c_str()));
+        devtools_proxy,
+        base::StringPrintf("Request method must be GET, but is %s",
+                           method_str.as_string().c_str()));
     return false;
   }
   out->set_request_method(method_str);