Change `paths-ignore` to `paths` in `markdownlint.yml` (#2211)

Markdown lint should only run when a `*.md` file has changed. This was likely a copy paste mistake when this Github action was created.

.github/workflows/markdownlint.yml

@@ -3,7 +3,7 @@ name: markdownlint
 on:
   push:
     branches: [ main, metrics ]
-    paths-ignore:
+    paths:
     - '**.md'
   pull_request:
     branches: [ main, metrics ]