fix: Update collector tag in example to latest

Follow-up from open-telemetry#1383. Debug exporter isn't available until v0.86.0, so this new configuration throws an error when trying to run the collector. `Error: cannot unmarshal the configuration: unknown exporters type "debug" for "debug"` Instead of setting to v0.86.0, I noticed the other docker files omitted the tag (so latest is pulled). I just updated this to match.

examples/traces/demo/docker-compose.yml

@@ -41,7 +41,7 @@ services:
       OTEL_SERVICE_NAME: service-three
 
   collector:
-    image: otel/opentelemetry-collector-contrib:0.39.0
+    image: otel/opentelemetry-collector-contrib
     volumes:
       - './collector:/etc/otel'
     ports: