Fix isSetextHeading (#19912)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/language-markdown/utilities.js

@@ -317,8 +317,9 @@ function hasPrettierIgnore(path) {
 }
 
 function isSetextHeading(node) {
-  const { start, end } = node.position;
-  return start.line !== end.line;
+  return (
+    node.type === "heading" && node.position.start.line < node.position.end.line
+  );
 }
 
 const isNewLine = (node) => node?.type === "whitespace" && node.value === "\n";