fix sign of delta of release_memory tracer event (#16)

trace.go

@@ -261,7 +261,7 @@ func (t *trace) ReleaseMemory(scope string, size, mem int64) {
 	t.push(traceEvt{
 		Type:   traceReleaseMemoryEvt,
 		Scope:  scope,
-		Delta:  -size,
+		Delta:  size,
 		Memory: mem,
 	})
 }