Docs/admin-guide: Remove pmf leftover reference from the index

Docs/admin-guide: Remove pmf leftover reference from the index

pmf.rst was removed by the commit 2fd66f7d3b0d ("platform/x86/amd/pmf:
Remove update system state document") but the reference in the
admin-guide index remained in place which triggers this warning:

Documentation/admin-guide/index.rst:75: WARNING: toctree contains
reference to nonexisting document 'admin-guide/pmf'

Remove pmf also from the index to avoid the warning.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20240715104102.4615-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>