fix test script path in CONTRIBUTING.md

`./bin/ruby-petstore.sh` renamed by OpenAPITools#534 so we should fix CONTRIBUTING.md

CONTRIBUTING.md

@@ -83,7 +83,7 @@ For [Vendor Extensions](https://github.com/OAI/OpenAPI-Specification/blob/master
 To add test cases (optional) covering the change in the code generator, please refer to [modules/openapi-generator/src/test/java/org/openapitools/codegen](https://github.com/openapitools/openapi-generator/tree/master/modules/openapi-generator/src/test/java/org/openapitools/codegen)
 
 To test the templates, please perform the following:
-- Update the Petstore sample by running the shell script under `bin` folder. For example, run `./bin/ruby-petstore.sh` to update the Ruby PetStore API client under [`samples/client/petstore/ruby`](https://github.com/openapitools/openapi-generator/tree/master/samples/client/petstore/ruby) For Windows, the batch files can be found under `bin\windows` folder. (If you find that there are new files generated or unexpected changes as a result of the update, that's not unusual as the test cases are added to the OpenAPI spec from time to time. If you've questions or concerns, please open a ticket to start a discussion)
+- Update the Petstore sample by running the shell script under `bin` folder. For example, run `./bin/ruby-client-petstore.sh` to update the Ruby PetStore API client under [`samples/client/petstore/ruby`](https://github.com/openapitools/openapi-generator/tree/master/samples/client/petstore/ruby) For Windows, the batch files can be found under `bin\windows` folder. (If you find that there are new files generated or unexpected changes as a result of the update, that's not unusual as the test cases are added to the OpenAPI spec from time to time. If you've questions or concerns, please open a ticket to start a discussion)
 - Run the tests in the sample folder, e.g. in `samples/client/petstore/ruby`, run `mvn integration-test -rf :RubyPetstoreClientTests`. (some languages may not contain unit testing for Petstore and we're looking for contribution from the community to implement those tests)
 - Finally, git commit the updated samples files: `git commit -a`
   (`git add -A` if added files with new test cases)