fix: Signed headers list in AWS example

All the headers in the example, except for Authorization itself should be included on the signed header list.

Author: amanda-tarafa

aip/auth/4117.md

@@ -277,7 +277,7 @@ The auth libraries and applications **must** follow the steps below:
         "key": "x-amz-date"
       },
       {
-        "value": "AWS4-HMAC-SHA256 Credential=AKIASOZTBDV4D7ABCDEDF/20200228/us-east-1/sts/aws4_request, SignedHeaders=host;x-amz-date,Signature=abcedefdfedfd",
+        "value": "AWS4-HMAC-SHA256 Credential=AKIASOZTBDV4D7ABCDEDF/20200228/us-east-1/sts/aws4_request, SignedHeaders=host;x-amz-date;x-amz-security-token;x-goog-cloud-target-resource, Signature=abcedefdfedfd",
         "key": "Authorization"
       },
       {