Fix Example readme file to use correct commands for local testing (#9582)

Author: cijothomas

examples/demo/README.md

@@ -44,10 +44,10 @@ Developers interested in running a local build of the Collector need to build a
 docker image using the command below:
 
 ```shell
-make docker-otelcol
+make docker-otelcontribcol
 ```
 
-And set an environment variable `OTELCOL_IMG` to `otelcol:latest` before 
+And set an environment variable `OTELCOL_IMG` to `otelcontribcol` before 
 launching the command `docker-compose up -d`.