network_traffic: fix documentation for flows period

[efd6]

What does this PR do?

This fixes documentation that misdirects users to use "-1" as a sentinel. This value causes packetbeat to fail to configure.

See https://github.com/elastic/beats/blob/25acf60399e2c56f544f4f76cb76611a28847cdf/packetbeat/flows/flows.go#L49-L66 and https://play.golang.com/p/Z7Si8WEzDAL.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• For [Network Packet Capture] Flow Metrics not reported properly  #5771

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• Start Time: 2023-04-05T22:18:59.918+0000

• Duration: 71 min 27 sec

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Package network_traffic - 1.10.1 containing this change is available at https://epr.elastic.co/search?package=network_traffic