update url for rest-api

[donhui]

Now, the docs for rest-api plugin point to the pipeline-stage-view-plugin/README.md in plugins site.
Actually, it should point to the rest-api/README.md.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[donhui]

@olamy Can you help review this pr? thanks.