bpf: fix typo in header for bpf_perf_prog_read_value

Fix a simple typo in the documentation for bpf_perf_prog_read_value.

Signed-off-by: Florian Lehner <dev@der-flo.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20230203121439.25884-1-dev@der-flo.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>