docs/pmu: Fix toctree
docs/pmu: Fix toctree

Warnings reported by Sphinx v1.7.8:
/home/docs/checkouts/readthedocs.org/user_builds/envytools/checkouts/latest/docs/nvrm/pmu/index.rst:9: WARNING: toctree contains reference to nonexisting document 'nvrm/pmu/ucode_cmds'
checking consistency... /home/docs/checkouts/readthedocs.org/user_builds/envytools/checkouts/latest/docs/nvrm/pmu/ucode-cmds.rst: WARNING: document isn't included in any toctree

envytools/envytools,envytools/envytools,karolherbst/envytools,karolherbst/envytools,envytools/envytools,envytools/envytools,karolherbst/envytools,envytools/envytools,karolherbst/envytools,karolherbst/envytools