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)

• 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

• This PR is translated from:
• Other reference link(s):

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-01 01:39:56.352818216 +0000 UTC m=+1202122.838307048: ☑️ agreed by csuzhangxc.
• 2024-07-09 03:21:17.079352568 +0000 UTC m=+327774.314586682: ☑️ agreed by Oreoxmt.
• 2024-07-09 03:21:23.081274712 +0000 UTC m=+327780.316508824: ☑️ agreed by Oreoxmt.

[Oreoxmt]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.3: #2589.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.4: #2590.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.5: #2591.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.6: #2592.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-1.2: #2593.