Use Markdown when use k8s-api-version variable

[peterzhongyi]

#3962 was submitted to fix the broken CI: https://pantheon.corp.google.com/cloud-build/builds;region=global/ee067e2c-418a-4be3-869e-1cdc31a777fc;step=19?e=13803378&mods=allow_workbench_image_override&project=agones-images

Later #3963 was submitted to use k8s-api-version using string literal.

This PR achieves the same effect as #3963 with Markdown syntax since the files are .md files.