Docs: Add ttl examples to output aws_dynamodb

[adam-knights]

Update the docs for aws_dynamodb output to have examples for ttl.

As a beginner to Benthos but familiar with DynamoDb I assumed I needed to enter a Unix timestamp myself. I later viewed the cache processor and stepped into the code and realized it needed to be like this for parse duration. Hopefully this helps someone new too :).

[CLAassistant]

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