Misleading ValheimPlus link in README

The ValheimPlus link in the README

> [ValheimPlus](https://github.com/valheimPlus/ValheimPlus) is a popular Valheim mod based on BepInEx.

 points to the unmaintained https://github.com/valheimPlus/ValheimPlus rather than the maintained fork https://github.com/Grantapher/ValheimPlus that seems to be used by default i.e. `VALHEIM_PLUS_REPO=${VALHEIM_PLUS_REPO:-Grantapher/ValheimPlus}`.