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