[journald] Update package description

[andrewkroh]

What does this PR do?

Set description to "Collect logs from journald with Elastic Agent."

Relates: #1901

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.

Related issues

• Relates: Update title and description based on guidelines for Elastic Agent based Observability integrations #1901

[elasticmachine]

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

[elasticmachine]

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• Start Time: 2021-12-09T14:05:51.935+0000

• Duration: 12 min 25 sec

• Commit: f924146

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Total	3

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