doc(carousel): using getImgUrl function in customPaging demo

[dyh333]

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This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
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What's the background?

1. Describe the source of requirement.
2. Resolve what problem.
3. Related issue link.

API Realization (Optional if not new feature)

优化carousel中demo的代码写法

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2. What will say in changelog?
3. Does this PR contains potential break change or other risk?

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Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
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