Doc/soc/intel/mp_init: Mark up Reference section title as title

It’s a section title, so mark it up as a title as it’s done similarily
in other documents.

Change-Id: If9d524afe6f80ae1b2704d11617786ee923814b2
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65215
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

Documentation/soc/intel/mp_init/mp_init.md

@@ -51,6 +51,6 @@ option in order to perform SGX and C6DRAM enabling.
 Typically all platforms supported by FSP 2.1 specification will have
 external PPI service feature implemented.
 
-[References]
+## References
 - [PPI](../fsp/ppi/ppi.md)
 - [MP Service PPI](../fsp/ppi/mp_service_ppi.md)