netflow: fix sample event

[efd6]

What does this PR do?

This fixes the event.category field in the sample event.

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

See #3016 (comment)

Related issues

• Closes netflow using invalid field values according to ECS #3047

Screenshots

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• Start Time: 2022-05-12T09:10:09.250+0000

• Duration: 14 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	92
Skipped	0
Total	92

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 3.623
Classes	100.0% (1/1)	💚 3.623
Methods	100.0% (7/7)	💚 11.799
Lines	100.0% (56/56)	💚 11.058
Conditionals	100.0% (0/0)	💚