internal/trace/tracev2: fix EvSTWBegin comment to include stack ID

Change-Id: I6a6a69644fb9a6e765933384cdb17c63458be69a
Reviewed-on: https://go-review.googlesource.com/c/go/+/694617
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: felixge

src/internal/trace/tracev2/events.go

@@ -48,7 +48,7 @@ const (
 	EvGoStatus            // goroutine status at the start of a generation [timestamp, goroutine ID, thread ID, status]
 
 	// STW.
-	EvSTWBegin // STW start [timestamp, kind]
+	EvSTWBegin // STW start [timestamp, kind, stack ID]
 	EvSTWEnd   // STW done [timestamp]
 
 	// GC events.