docs: psi: use correct config name

Commit 2ce7135a ("psi: cgroup support") adds documentation which refers
to CONFIG_CGROUP, but the correct name is CONFIG_CGROUPS.

Correct the reference to CONFIG_CGROUPS.

Co-developed-by: Sabina Trendota <sabinatrendota@gmail.com>
Signed-off-by: Sabina Trendota <sabinatrendota@gmail.com>
Signed-off-by: Ramazan Safiullin <ram.safiullin2001@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230728115600.231068-1-ram.safiullin2001@gmail.com