[master] Format markdown (#1838)
[master] Format markdown (#1838)

Produced via:
  `prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github | grep -v docs/cmd/)`
/assign chaodaig chizhg
/cc chaodaig chizhg
knative/test-infra,knative/test-infra,knative/test-infra