perf_event_open.2: tfix

Reported-by: 尹杰 <writalnaie@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/perf_event_open.2

@@ -1190,7 +1190,7 @@ struct read_format {
     u64 nr;            /* The number of events */
     u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
     u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
-    struct
+    struct {
         u64 value;     /* The value of the event */
         u64 id;        /* if PERF_FORMAT_ID */
     } values[nr];