fix: Transfer header checkbox label unit calculating issue

[morenyang]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

💡 Background and solution

The label for checkbox in the header of Transfer component shows the text like this:

ant-design/components/transfer/list.tsx

Lines 313 to 316 in 0b4caf9

	<span>
	{(checkedKeys.length > 0 ? `${checkedKeys.length}/` : '') + filteredItems.length}{' '}
	{unit}
	</span>

total items displaying use variable filteredItems.length, but calculate unit use dataSource.length:

ant-design/components/transfer/list.tsx

Line 281 in 0b4caf9

const unit = dataSource.length > 1 ? itemsUnit : itemUnit;

and it may cause a case like 1 items:

ant-design/components/transfer/__tests__/index.test.js

Line 249 in 0b4caf9

expect(headerText(wrapper)).toEqual('1 items');

but expect is 1 item.

📝 Changelog

Language	Changelog
🇺🇸 English	Fix Transfer header checkbox label unit calculating issue.
🇨🇳 Chinese

☑️ 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
• Changelog is provided or not needed

[tests-checker]

Could you please add tests to make sure this change works as expected?

[ant-design-bot]

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[codecov]

Codecov Report

Merging #21136 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #21136   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files         296      296           
  Lines        6912     6912           
  Branches     1919     1919           
=======================================
  Hits         6736     6736           
  Misses        176      176

Impacted Files	Coverage Δ
components/transfer/list.tsx	100% <100%> (ø)	⬆️

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