Update README.md with information about invalid variable key

1 job for !20453 with patch-38 in 1 minute and 2 seconds (queued for 5 seconds)
latest detached
Status Job ID Name Coverage
  Test
failed #355975551
gitlab-org
docs lint

00:01:02

 
Name Stage Failure
failed
docs lint Test
=> Checking /builds/kki2/gitlab/doc for executable permissions...
=> Checking for new README.md files...
✔ Linting passed
$ markdownlint --config .markdownlint.json 'doc/**/*.md'
doc/ci/variables/README.md: 195: MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```"]
Running after script...
$ date
Wed Nov 20 10:45:44 UTC 2019
ERROR: Job failed: exit code 1