Error in guides.rubyonrails.org

I am fairly new to Rails, but I think I've discovered an error in one
of the doc pages. Who do I tell about it?

You can post it here in this ML. Or open an issue on the Rails issue tracker in GitHub - https://github.com/rails/rails/issues

Not sure of your background, but the preferred way to handle this is to fork the docs, make the suggested change, and then send a pull request. If you’re familiar with git, then this is pretty straightforward. If you’re not familiar with git, this might be a good first step, since the whole ruby on rails community ecosystem is built on git.

You didn’t mention where you were looking to see the error. But the rails docs are viewable here: http://guides.rubyonrails.org/ and the bottom of that page points to the rails docs source here https://github.com/lifo/docrails

Yes, you can just open an issue and have someone else do the work. But let me urge you, please, to try to make life easier for those many hard-working members of the rails core team and send them a pull request.