Docs/mm/index: Remove 'Memory Management Guide' chapter marker

'Memory Management Guide' chapter aims to be not an additional chapter
of the document, but the ultimate single outline of the document.  In
the sense, marking it as a chapter under the document makes no sense,
and the rendered document looks odd.  Remove the chapter marker.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240701190512.49379-3-sj@kernel.org