Add pending PR to change log
Adds PR to pending change log.
### SDK Features
### SDK Enhancements
+* `example/service/dynamodb`: Add custom unmarshaller error example for TransactWriteItems ([#2548](https://github.com/aws/aws-sdk-go/pull/2548))
+ * Adds an example for building and using a custom unmarshaller to unmarshal TransactionCancelledExceptions from the error response of TransactWriteItems operation.
### SDK Bugs
