selftests/bpf: Fix trivial typo

Fix trivial typo in comment from 'oveflow' to 'overflow'.

Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211122070528.837806-1-dfustini@baylibre.com