Documentation: cgroup-v2: fix path to example BPF program

This file has been moved into the "progs" subdirectory, together with all
test BPF programs.

Fixes: bd4aed0e ("selftests: bpf: centre kernel bpf objects under new subdir "progs"")
Signed-off-by: Antonio Terceiro <antonio.terceiro@linaro.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jiong Wang <jiong.wang@netronome.com>
Link: https://lore.kernel.org/r/20210224131631.349287-1-antonio.terceiro@linaro.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/cgroup-v2.rst

@@ -2094,7 +2094,7 @@ If the program returns 0, the attempt fails with -EPERM, otherwise
 it succeeds.
 
 An example of BPF_CGROUP_DEVICE program may be found in the kernel
-source tree in the tools/testing/selftests/bpf/dev_cgroup.c file.
+source tree in the tools/testing/selftests/bpf/progs/dev_cgroup.c file.
 
 
 RDMA