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

Documentation/accounting/psi.rst

@@ -178,7 +178,7 @@ Userspace monitor usage example
 Cgroup2 interface
 =================
 
-In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem
+In a system with a CONFIG_CGROUPS=y kernel and the cgroup2 filesystem
 mounted, pressure stall information is also tracked for tasks grouped
 into cgroups. Each subdirectory in the cgroupfs mountpoint contains
 cpu.pressure, memory.pressure, and io.pressure files; the format is