RandOptGen reference is in the implementation

[kvdblom]

In problems.yaml the reference (doi link) for gen_randoptgen is wrongly placed in impl_randoptgen instead of in gen_randoptgen itself

This should be moved to gen_randoptgen:

  - type: example
    url: https://doi.org/10.1145/3712256.3726478

and changed to:

references:
  - link
      type: doi
      url: https://doi.org/10.1145/3712256.3726478