network_traffic: fix typo

[efd6]

What does this PR do?

This fixes a typo.

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

elastic-package test in network_traffic.

Related issues

• Relates fields yaml files typos #2643

Screenshots

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Start Time: 2022-02-08T21:20:23.962+0000

• Duration: 75 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	134
Skipped	0
Total	134

🤖 GitHub comments

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[andrewkroh]

I think we should remove default_field (and default_fields) entirely because it has no use within Fleet packages.

[efd6]

I'll do that for this package and follow up for others with an issue.