Fix: typo in prometheus version (#561)

Author: mayurkale22

packages/opencensus-exporter-prometheus/src/prometheus-stats.ts

@@ -39,7 +39,7 @@ export class PrometheusStatsExporter implements StatsEventListener {
   static readonly DEFAULT_OPTIONS = {
     port: 9464,
     startServer: false,
-    contentType: 'text/plain; text/plain; version=3; charset=utf-8',
+    contentType: 'text/plain; text/plain; version=0.0.13; charset=utf-8',
     prefix: ''
   };