Improve markdown lint check (#2497)

Author: trask

.mise/tasks/lint/markdown.sh

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 #MISE description="Lint markdown files"
-#MISE flag "--fix" help="Automatically fix issues"
+#USAGE flag "--fix" help="Automatically fix issues"
 
 set -e
 
-if [ "${usage_fix}" = "true" ]; then
+if [ "${usage_fix:-false}" = "true" ]; then
   markdownlint-cli2 --fix "**/*.md" "#**/build" "#**/node_modules" "#CHANGELOG.md" "#ibm-mq-metrics/docs/metrics.md" "#.github/pull_request_template.md"
 else
   markdownlint-cli2 "**/*.md" "#**/build" "#**/node_modules" "#CHANGELOG.md" "#ibm-mq-metrics/docs/metrics.md" "#.github/pull_request_template.md"