cmd/test2json: document time.Time JSON format

This is already documented in the time.Time package
but people might not look there.

Followup to CL 76872, which I submitted accidentally
(Gerrit has placed the Submit button next to Reply again.)

Change-Id: Ibfd6a4da241982d591a8698282a0c15fe9f2e775
Reviewed-on: https://go-review.googlesource.com/77010
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/test2json/main.go

@@ -24,7 +24,7 @@
 // corresponding to the Go struct:
 //
 //	type TestEvent struct {
-//		Time    time.Time
+//		Time    time.Time // encodes as an RFC3339-format string
 //		Event   string
 //		Package string
 //		Test    string