docs: fix README to show correct generated sources path

- Updated README.md to use `src/gen/java` instead of outdated `src/gen/java/main`
- Keeps documentation consistent with latest client generation setup

Author: bsayli

customer-service-client/README.md

@@ -45,7 +45,7 @@ mvn clean install
 Generated sources will be placed under:
 
 ```
-target/generated-sources/openapi/src/gen/java/main
+target/generated-sources/openapi/src/gen/java
 ```
 
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