DM: Revise wrong syntax for command example on get into the pod in eng doc (#2587)

[ti-chi-bot]

This is an automated cherry-pick of #2587

#2586 Example syntax error in eng doc.

What is changed, added, or deleted? (Required)

Add a hyphen on kubectl command: from kubectl exec -it - to kubectl exec -it --
because - is not valid syntax, and will throw an error message. Detail inside the #2586

Which TiDB Operator version(s) do your changes apply to? (Required)

• master (the latest development version)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)
• v1.2 (TiDB Operator 1.2 versions)

What is the related PR or file link(s)?

NA

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[Oreoxmt]

/approve

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