book: fix link for platform support
- [controller-gen CLI](./reference/controller-gen.md)
- [completion](./reference/completion.md)
- [Artifacts](./reference/artifacts.md)
- - [Platform Support](platform.md)
+ - [Platform Support](./reference/platform.md)
- [Configuring EnvTest](./reference/envtest.md)
- [Metrics](./reference/metrics.md)
