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

[Jingyuma]

#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)

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

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

NA

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[LGTM Timeline notifier]

Timeline:

• 2024-07-01 01:39:56.352818216 +0000 UTC m=+1202122.838307048: :ballot_box_with_check: agreed by csuzhangxc.