trace2: increment event format version

In 64bc752 (trace2: add trace2_child_ready() to report on background
children, 2021-09-20), we added a new "child_ready" event. In
Documentation/technical/api-trace2.txt, we promise that adding a new
event type will result in incrementing the trace2 event format version
number, but this was not done. Correct this in code & docs.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2/tr2_tgt_event.c

@@ -20,7 +20,7 @@ static struct tr2_dst tr2dst_event = { TR2_SYSENV_EVENT, 0, 0, 0, 0 };
  * a new field to an existing event, do not require an increment to the EVENT
  * format version.
  */
-#define TR2_EVENT_VERSION "2"
+#define TR2_EVENT_VERSION "3"
 
 /*
  * Region nesting limit for messages written to the event target.