doc/cgroup: Fix title underline length

Commit b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting
cgroup") introduced a new documentation file, with a shorter than
expected underline. This results in a documentation build warning. Fix
that underline length.

Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20250113154611.624256bf@canb.auug.org.au/
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113092608.1349287-4-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>