Updates to demo docs to reflect root README container image name

[pdxjohnny]

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
  • N/A
• [x] Link to relevant pull requests, esp. upstream and downstream changes
  • https://github.com/jenkinsci/jenkinsfile-runner/issues/650
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue
  • N/A