docs: mm: numaperf.rst: get rid of a build warning

When building it, it gets this warning:

	Documentation/admin-guide/mm/numaperf.rst:168: WARNING: Footnote [1] is not referenced.

The problem is that this is not really a reference, as it is not
mentioned within the documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>