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

Documentation/mm/allocation-profiling.rst

@@ -46,7 +46,6 @@ Example output::
          55M     4887 mm/slub.c:2259 func:alloc_slab_page
         122M    31168 mm/page_ext.c:270 func:alloc_page_ext
 
-===================
 Theory of operation
 ===================