docs: Missing "plugins" key on the samples for "how to write a plugin"

### Which packages are impacted by your issue?

@graphql-codegen/cli

### Describe the bug

On https://the-guild.dev/graphql/codegen/docs/custom-codegen/plugin-structure the yaml config files are missing the "plugin" key so all the demos fail if run as is. Fixed the code samples to show what I think is hte right structure as taken from the runable demos on https://the-guild.dev/graphql/codegen

### Your Example Website or App

Its just docs...

### Steps to Reproduce the Bug or Issue

Cut and paste in the config and try to run the code-gen as per the docs and you will get a config is invalid error.

### Expected behavior

Running the demo code locally shouldn't give you an error about the config being wrong

### Screenshots or Videos

_No response_

### Platform

- OS: [e.g. macOS, Windows, Linux]
- NodeJS: [e.g. 18.5.0]
- `graphql` version: [e.g. 16.3.0]
- `@graphql-codegen/*` version(s): [e.g. 2.6.2]

### Codegen Config File

_No response_

### Additional context

_No response_