Fix rubocop warnings in Ruby client's custom spec

Created by: autopp

This PR fixes Rubocop warnings in samples/openapi3/client/petstore/ruby.

Before:

$ (cd samples/openapi3/client/petstore/ruby && rubocop -c .rubocop.yml)
Inspecting 129 files
............................................................................C..................................................C.

Offenses:

spec/custom/pet_spec.rb:82:9: C: Layout/LeadingCommentSpace: Missing space after #.
        #expect(e.message).to eq("Error message: the server returns an error\nHTTP status code: 404\nResponse headers: {\"Date\"=>\"Tue, 26 Feb 2019 04:35:40 GMT\", \"Access-Control-Allow-Origin\"=>\"*\", \"Access-Control-Allow-Methods\"=>\"GET, POST, DELETE, PUT\", \"Access-Control-Allow-Headers\"=>\"Content-Type, api_key, Authorization\", \"Content-Type\"=>\"application/json\", \"Connection\"=>\"close\", \"Server\"=>\"Jetty(9.2.9.v20150224)\"}\nResponse body: {\"code\":1,\"type\":\"error\",\"message\":\"Pet not found\"}")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
spec/petstore_helper.rb:51:1: C: Layout/TrailingEmptyLines: 2 trailing blank lines detected.

129 files inspected, 2 offenses detected

After:

$ (cd samples/openapi3/client/petstore/ruby && rubocop -c .rubocop.yml)
Inspecting 129 files
.................................................................................................................................

129 files inspected, no offenses detected

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language. @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)