Update linter.yml (#8011)

Update syntax to disable Markdown check

Author: dshevtsov

.github/workflows/linter.yml

@@ -54,4 +54,4 @@ jobs:
           VALIDATE_ALL_CODEBASE: false
           DEFAULT_BRANCH: master
           VALIDATE_HTML: false
-          VALIDATE_MD: false
+          VALIDATE_MARKDOWN: false