The document at GitHub Pages is old

[kmyk]

The document at GitHub Pages of this repository is tracking production branch. Is this intended?
I recommend to use master for GitHub Pages, or make clear what branches are used (e.g. use https://atcoder.github.io/ac-library/production/document_en/ as the URL).

For example, the fix for #2 is merged into master but not merged into production yet, and the document at GitHub Pages is still wrong.

[yosupo06]

We changed the URL so that it contains branch/tag name (In later, we'll change the link of README)