cdc: fix index-value dispatcher config typo

[kennytm]

What is changed, added or deleted? (Required)

In this section https://docs.pingcap.com/tidb/stable/ticdc-sink-to-kafka#partition-dispatchers, the dispatcher config key index-name should have been index according to the actual implementation in pingcap/tiflow#9825.

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• [x] master (the latest development version)
• [x] v7.6 (TiDB 7.6 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [ ] v7.4 (TiDB 7.4 versions)
• [ ] v7.3 (TiDB 7.3 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions)
• [ ] v5.2 (TiDB 5.2 versions)
• [ ] v5.1 (TiDB 5.1 versions)
• [ ] v5.0 (TiDB 5.0 versions)

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

• This PR is translated from:
• Other reference link(s): #15278

Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[LGTM Timeline notifier]

Timeline:

• 2024-01-31 08:45:29.114659898 +0000 UTC m=+1555170.678957602: :ballot_box_with_check: agreed by Oreoxmt.

[Oreoxmt]

@3AceShowHand @nongfushanquan PTAL

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[Oreoxmt]

/cc @CharlesCheung96

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-01-31 08:45:29.114659898 +0000 UTC m=+1555170.678957602: :ballot_box_with_check: agreed by Oreoxmt.
• 2024-06-12 10:08:33.220934819 +0000 UTC m=+545667.274246740: :ballot_box_with_check: agreed by qiancai.

[Oreoxmt]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #17885.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17886.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17887.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #17888.