bpf: fix formatting for bpf_get_stack() helper doc

Fix formatting (indent) for bpf_get_stack() helper documentation, so
that the doc is rendered correctly with the Python script.

Fixes: c195651e

 ("bpf: add bpf_get_stack helper")
Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>