Update CHANGELOG.md

Author: manueliglesias

packages/aws-appsync/CHANGELOG.md

@@ -1,7 +1,7 @@
 # Changelog
 
 ### vNext
-- Update x-amz-user-agent header [PR#XX](https://github.com/awslabs/aws-mobile-appsync-sdk-js/pull/XX)
+- Update x-amz-user-agent header [PR#40](https://github.com/awslabs/aws-mobile-appsync-sdk-js/pull/40)
 
 ### 1.0.9
 - Fix AWS_IAM credentials fetching [PR#38](https://github.com/awslabs/aws-mobile-appsync-sdk-js/pull/38)