perf: Fix wrong comment about default event_idx

Since commit c719f560 ("perf: Fix and clean up initialization of
pmu::event_idx"), event_idx default implementation has returned 0, not
idx + 1, so fix the comment that can be misleading.
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>

include/linux/perf_event.h

@@ -445,7 +445,8 @@ struct pmu {
 
 	/*
 	 * Will return the value for perf_event_mmap_page::index for this event,
-	 * if no implementation is provided it will default to: event->hw.idx + 1.
+	 * if no implementation is provided it will default to 0 (see
+	 * perf_event_idx_default).
 	 */
 	int (*event_idx)		(struct perf_event *event); /*optional */