message: fix: openapi button add `download` attribute
diff: <h1>{{info.title}} <span class="api-info-version">({{info.version}})</span></h1>
  <p class="download-openapi" *ngIf="specUrl">
  Download OpenAPI (fka Swagger) specification:
  - <a class="openapi-button" target="_blank" attr.href='{{specUrl}}'> Download </a>
  + <a class="openapi-button" download target="_blank" attr.href='{{specUrl}}'> Download </a>
  </p>
  <p>
  <!-- TODO: create separate components for contact and license ? -->
  