fix docs for maximum delay_seconds

The maximum delay_seconds on a message is actually 15 minutes, however the aws-sdk-ruby has an example in the documentation of batch_send with of 1 hour delay, which fails to run:

```ruby
begin
  queue.batch_send("1", :delay_seconds => 3600)
rescue => ex
  ex.failures
end
=> [{:error_code=>"InvalidParameterValue", :error_message=>"Value 3600 for parameter DelaySeconds is invalid. Reason: DelaySeconds must be >= 0 and <= 900.", :sender_fault=>true}]