correct path in cumulus README

[SihanoukSolver29]

Description

This PR fixes the file path in cumulus/README.md so the link to the container documentation points to the correct location.

Review Notes

The only change are the links in cumulus/README.md from ./docs/contributor/container.md to ../docs/contributor/container.md.

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T required)
  • External contributors: ask maintainers to put the right label on your PR.
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[cla-bot-2021]

User @SihanoukSolver29, please sign the CLA here.