remove misleading "suggest changes" link

At the bottom of https://pvp.haskell.org/#related it says:

> If you'd like suggest changes, please [open an issue on GitHub](https://github.com/haskell/pvp/issues).

But that doesn't do anything in practice because changes suggested on github are only made on Github; they don't make it to the website. For example, PR #20 from 2018 contained a bugfix that corrects a nonsensical suggestion in the flowchart diagram, but the website still hasn't been updated.

In practical terms, there is no way to get changes into the published PVP, so the Github issue suggestion should be removed.