[googelcloudexporter] Fixed configuration path in README (open-telemetry#31404)

The google cloud exporter README suggests that the configfile is in
/etc/otel/config.yaml; but it's not.

(and I pulled my hair out for a while trying to figure out why my config
was not working).

**Description:**
I changed the path used for the config file in the example to reflect
the path used / set in the Dockerfile.

**Link to tracking Issue:**
open-telemetry#31403 

---------

Co-authored-by: Ziqi Zhao <[email protected]>

Author: 2 people

exporter/googlecloudexporter/README.md

@@ -56,9 +56,9 @@ When running the Collector in a Docker container, a credentials file can be pass
 
 ```
 docker run \
-  --volume ~/service-account-key.json:/etc/otel/key.json \
-  --volume $(pwd)/config.yaml:/etc/otel/config.yaml \
-  --env GOOGLE_APPLICATION_CREDENTIALS=/etc/otel/key.json \
+  --volume ~/service-account-key.json:/etc/otelcol-contrib/key.json \
+  --volume $(pwd)/config.yaml:/etc/otelcol-contrib/config.yaml \
+  --env GOOGLE_APPLICATION_CREDENTIALS=/etc/otelcol-contrib/key.json \
   --expose 4317 \
   --expose 55681 \
   --rm \