selftests/bpf: Fix typo in Makefile

The selftest build fails when trying to install the scripts:

rsync: [sender] link_stat "tools/testing/selftests/bpf/test_docs_build.sh" failed: No such file or directory (2)

Fix the filename.

Fixes: a01d935b ("tools/bpf: Remove bpf-helpers from bpftool docs")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210308182830.155784-1-jean-philippe@linaro.org