trace2: correct trace2 field name documentation

Correct the api-trace2 documentation, which lists "signal" as an
expected field for the signal event type, but which actually outputs
"signo" as the field name.

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

Documentation/technical/api-trace2.txt

@@ -668,7 +668,7 @@ completed.)
 	"event":"signal",
 	...
 	"t_abs":0.001227,  # elapsed time in seconds
-	"signal":13        # SIGTERM, SIGINT, etc.
+	"signo":13         # SIGTERM, SIGINT, etc.
 }
 ------------