scripts/bpf: Switch to more portable python3 shebang

Change "/usr/bin/python3" to "/usr/bin/env python3" for
more portable solution in bpf_helpers_doc.py.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200225205426.6975-1-scott.branden@broadcom.com