Fix position of envtest reference in book ToC
A bad merge or something screwed up the order of the ToC wrt to the
envtest reference section.  This fixes it.
- [controller-gen CLI](./reference/controller-gen.md)
- [Artifacts](./reference/artifacts.md)
- [Writing controller tests](./reference/writing-tests.md)
- - [Metrics](./reference/metrics.md)
- [Using envtest in integration tests](./reference/testing/envtest.md)
+ - [Metrics](./reference/metrics.md)
+
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[Appendix: The TODO Landing Page](./TODO.md)
