bpf: Fix reStructuredText markup

The patch fixes:
$ scripts/bpf_helpers_doc.py > bpf-helpers.rst
$ rst2man bpf-helpers.rst > bpf-helpers.7
bpf-helpers.rst:1105: (WARNING/2) Inline strong start-string without end-string.

Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200422082324.2030-1-jwilk@jwilk.net