fix($markdown-loader): always use / instead of \ in relPath

[shigma]

Summary

Always use / instead of \ in relPath.

Bad:

vuepress/packages/@vuepress/markdown-loader/index.js

Line 68 in 7848a30

relPath: path.relative(sourceDir, file)

Good:

vuepress/packages/@vuepress/core/lib/node/Page.js

Line 68 in 7848a30

this.relativePath = path.relative(context.sourceDir, filePath).replace(/\\/g, '/')

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• Bugfix
• Feature
• Code style update
• Refactor
• Docs
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• No

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The PR fulfills these requirements:

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• Related documents have been updated
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