index_generator.pagination_dir doesn't take effect

[uiolee]

Check List

• I have already read README.
• I have already searched existing issues and they are not help to me.
• I examined error or warning messages and it's difficult to solve.
• I am using the latest version of this repository.
• I am using the latest version of Hexo.
• My Node.js is matched the required version.

Describe the bug

hexo-generator-index/README.md

Lines 21 to 27 in f8ef61c

	``` yaml
	index_generator:
	path: ''
	per_page: 10
	order_by: -date
	pagination_dir: page
	```

according to README, pagination_dir can be set by index_generator.pagination_dir.
But from the code, index_generator.pagination_dir is invalid.

hexo-generator-index/lib/generator.js

Line 11 in f8ef61c

const paginationDir = config.pagination_dir || 'page';

Expected behavior

Code and documentation should match, avoid misleading users.

How to reproduce

Screenshots

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Environment information

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Additional context

No response