bpf: adjust path to trace_output sample eBPF program

The sample file was renamed from trace_output_kern.c to
trace_output.bpf.c in commit d4fffba4 ("samples/bpf: Change _kern
suffix to .bpf with syscall tracing program"). Adjust the path in the
documentation comment for bpf_perf_event_output.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20250610140756.16332-1-tklauser@distanz.ch


Signed-off-by: Alexei Starovoitov <ast@kernel.org>