Docs/mm/allocation-profiling: mark 'Theory of operation' as chapter

'Theory of operation' part of allocation-profiling document is
apparently a chapter.  However, it is mistakenly marked as a document
title.  As a result, rendered mm document index page shows two items for
the document.  Fix it to be marked as a chapter.

Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240701190512.49379-2-sj@kernel.org