Add shouldRetryCancel update to pending change long
Adds PR improvement for shouldRetryCancel to pending changelog.
### SDK Features
### SDK Enhancements
+* `aws/request: Improve error handling in shouldRetryCancel ([#2298](https://github.com/aws/aws-sdk-go/pull/2298))
+ * Simplifies and improves SDK's detection of HTTP request errors that should be retried. Previously the SDK would incorrectly attempt to retry `EHOSTDOWN` connection errors. This change fixes this, by using the `Temporary` interface when available.
### SDK Bugs
