docs(contribute): update branch name

[chenxsan]

Webpack has renamed the master branch to main.

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[snitin315]

Is .mdx being used somewhere here?

[chenxsan]

The plan is to migrate all .md on this repository gradually to .mdx for two reasons:

1. No need to keep both as .mdx can do whatever .md could but not vice versa.
2. It's possible to customize the components in .mdx to improve page performance, see customize Link for mdx #4412 for details. While no such improvement could be possible in .md.

[snitin315]

Makes sense 👍