[#179345034] Revert change that breaks image link
[#179345034] Revert change that breaks image link

- Revert "[#176100862] changes 'master' to 'main'."
- This reverts commit 2de775c6be01c9cdb90b0a6f9f5edfe7054f5da7.

cloudfoundry/docs-cloudfoundry-concepts