Update reference site link

[azurealstn]

https://docs.spring.io/spring-session/reference/current/index.html goes to a 404 page

Use https://docs.spring.io/spring-session/reference/index.html instead which is the current documentation site.

I don't know which file the path to the image is in. 😢 So, I registered an issue instead of a pull request.

[Marcus Hert Da Coregio]

Thanks @azurealstn, the link is now fixed. The current version is now incorrectly pointing to 2021.2.3, but that is going to be fixed as well.