[ninjatrace] Normalize json flag

Change-Id: Ib1c8e720a1a38325690b1f499a0c48138eda7878
diff --git a/cmd/ninjatrace/ninjatrace.go b/cmd/ninjatrace/ninjatrace.go
index a19616e..adff26f 100644
--- a/cmd/ninjatrace/ninjatrace.go
+++ b/cmd/ninjatrace/ninjatrace.go
@@ -25,7 +25,7 @@
 
 var (
 	filename   = flag.String("filename", ".ninja_log", "filename of .ninja_log")
-	traceJSON  = flag.String("trace_json", "trace.json", "output filename of trace.json")
+	traceJSON  = flag.String("trace-json", "trace.json", "output filename of trace.json")
 	cpuprofile = flag.String("cpuprofile", "", "file to write cpu profile")
 )