{,x-pack/}packetbeat: 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

beats/packetbeat/flows/flows.go

Lines 49 to 66 in 25acf60

	duration := func(s string, d time.Duration) (time.Duration, error) {
	if s == "" {
	return d, nil
	}
	return time.ParseDuration(s)
	}
	timeout, err := duration(config.Timeout, defaultTimeout)
	if err != nil {
	logp.Err("failed to parse flow timeout: %v", err)
	return nil, err
	}
	period, err := duration(config.Period, defaultPeriod)
	if err != nil {
	logp.Err("failed to parse period: %v", err)
	return nil, err
	}

and https://play.golang.com/p/Z7Si8WEzDAL.

Why is it important?

The current documentation misleads users to use an invalid value.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

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

Use cases

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Logs

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• Start Time: 2023-04-04T03:27:32.707+0000

• Duration: 48 min 34 sec

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Test	Results
Failed	0
Passed	1742
Skipped	19
Total	1761

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