fix pagination

[lhz960904]

Summary
fix again pagination calculates the wrong intervallers, eg. when I have 2 posts, I pass perPagePosts is 1. but the pagination will calculates theintervallers is [ [ 0, 0 ], [ 1, 1 ] ] instead of [ [ 0, 1 ], [ 1, 2 ] ]

const path = require('path')

module.exports = {
  name: 'vuepress-theme-ihaoze',
  plugins: [
    '@vuepress/blog',
    ['@vuepress/pagination', {
      perPagePosts: 1
    }],
    [
      '@vuepress/register-components', {
      componentsDir: [
        path.resolve(__dirname, 'components')
      ]
    }]
  ]
}

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
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If changing the UI of default theme, please provide the before/after screenshot:

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• No

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The PR fulfills these requirements:

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Other information:

[ulivz]

Thanks