Fix the example of tidb's equivalent function for oracle's nvl2

[hfxsd]

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• This PR is translated from:Fix the example of tidb's equivalent function for oracle's nvl2 docs-cn#16162
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-12 09:07:37.984551126 +0000 UTC m=+607047.568804814: ☑️ agreed by Oreoxmt.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16143.