broken link: PP3, variational bayes

[stilxam]

in probabilistic programming lecture 3, the link to the constrains specification documentation of rxinfer provided (https://docs.rxinfer.com/stable/manuals/constraints-specification/) is broken.
I think it was moved to https://docs.rxinfer.com/stable/manuals/variational/constraints-specification/

[fonsp]

Thanks! I was already working on a fix :) #284