Fix crontab modification not properly relying on the cron_name parameter

[information-redacted]

• Pull request is based on the default branch (3.x at this time)
• Pull request includes tests for any new/updated operations/facts
• Pull request includes documentation for any new/updated operations/facts
• Tests pass (see scripts/dev-test.sh)
• Type checking & code style passes (see scripts/dev-lint.sh)

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I stumbled upon this issue because in my environment, the assert present in get_command was not hitting, and the result was an invalid sed command.

[Fizzadar]

Thanks for fixing this, @information-redacted!