Fix javadoc in clioption

Created by: jmini

See issue #63 (closed).

A new warning was introduced with #160:

https://github.com/OpenAPITools/openapi-generator/blob/bd50d368eb871dcdc4d7e9dd1b4c97ec77b23e3a/modules/openapi-generator/src/main/java/org/openapitools/codegen/CliOption.java#L105-L115

I have fixed it and I have now configured the build to fail when new warnings are created.