[typo] --split-by option must be passed to circleci

[jmatsu]

Description

I found a typo in an example of parallelism tests. --split-by option must be passed to circleci command but be wrongly included by --command option.

Reasons

A link to a GitHub and/or JIRA issue (if applicable). Otherwise, a brief sentence about why you made these changes.

Content Checklist

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[rosieyohannan]

@jmatsu Thank you so much for spotting this, and fixing it! 🎉