[cmd/opampsupervisor] fix docs for examples configuration dir (open-t…

…elemetry#31432)

**Description:** <Describe what has changed.>
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cmd/opampsupervisor/README.md

@@ -18,7 +18,7 @@ The supervisor is currently undergoing heavy development and is not ready for an
 3. Run the supervisor, substituting `<OS>` for your platform:
 
  ```shell
- go run . --config testdata/supervisor_<OS>.yaml
+ go run . --config examples/supervisor_<OS>.yaml
  ```
 
 4. The supervisor should connect to the OpAMP server and start a Collector instance.