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 d4fffba4d04b ("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>

include/uapi/linux/bpf.h

@@ -2409,7 +2409,7 @@ union bpf_attr {
  * 		into it. An example is available in file
  * 		*samples/bpf/trace_output_user.c* in the Linux kernel source
  * 		tree (the eBPF program counterpart is in
- * 		*samples/bpf/trace_output_kern.c*).
+ *		*samples/bpf/trace_output.bpf.c*).
  *
  * 		**bpf_perf_event_output**\ () achieves better performance
  * 		than **bpf_trace_printk**\ () for sharing data with user