Outdated doc for ACP on CodeCompanion integartion

I was following the docs to use codecompanion with opencode, noticed that it got slightly outdated. Decided to propose an update while at it. 

There was a braking change introduced in [CHANGELOG](https://github.com/olimorris/codecompanion.nvim/blob/c873ef7235687904e77da7b7656ec50602e8f2cd/CHANGELOG.md?plain=1#L112). And one link to env api settings got updated.
