MAINTAINERS: Set the field name for subsystem profile section

Subsystem profile section entry identifier is not having its field name
that can be parsed by maintainers_include.py, unlike other sections
which have their own human-readable field names.  As a result, profile
sections on rendered rst file is having weird name, 'P:'.  Set the field
name as 'Subsystem Profile'.

Fixes: 4699c504 ("Maintainer Handbook: Maintainer Entry Profile")
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240216201902.10095-1-sj@kernel.org

MAINTAINERS

@@ -24,7 +24,7 @@ Descriptions of section entries and preferred order
 	   filing info, a direct bug tracker link, or a mailto: URI.
 	C: URI for *chat* protocol, server and channel where developers
 	   usually hang out, for example irc://server/channel.
-	P: Subsystem Profile document for more details submitting
+	P: *Subsystem Profile* document for more details submitting
 	   patches to the given subsystem. This is either an in-tree file,
 	   or a URI. See Documentation/maintainer/maintainer-entry-profile.rst
 	   for details.