scsi: docs: arcmsr: Use a chapter heading for clarity

Add a chapter heading so that the document sections are not all
at the same level, mucking up the SCSI subsystem contents.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230518212749.18266-4-rdunlap@infradead.org
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Documentation/scsi/arcmsr_spec.rst

@@ -1,3 +1,4 @@
+===================
 ARECA FIRMWARE SPEC
 ===================