> Can someone indicate the place where such descriptions are hosted ?

MELPA derives the metadata both from a recipe file [1] and from the package
header [2].  In this case the package header appears to be at fault as it
mentions the wrong URL.  The flow for contributions to GitLab-hosted
repositories is signing up, forking the repository, creating a new branch and
commit fixing the issue, pushing it, then submitting a "Merge Request" for the
maintainer to review.  However I'd consider to just submit an issue or if you
cannot be bothered, sending them an email.

Vasilij

[1]: https://github.com/melpa/melpa/blob/master/recipes/hercules
[2]: https://gitlab.com/jjzmajic/hercules.el/-/blob/557da39878d0637395fdded91243b340c37eff7b/hercules.el#L5-10