Reference documentation is not up to date

[julienrf]

Currently, the Scala 3 reference documentation shows version 3.1.2 instead of the latest 3.1.3.

I think there is a missing step in the release process. The documentation should have been updated with the content of the main branch.

[Kordyjan]

The documentation is updating as it should. Only the version string is incorrect. I fixed it for now. After my vacation, I will think about a more robust solution. Maybe we can use previousDottyVersion here.

[Kordyjan]

It is now part of the release steps. I'll probably automate it further in the future.