fix: parse heading error (docsifyjs#2526)

Author: sy-records

src/plugins/search/markdown-to-txt.js

@@ -59,7 +59,7 @@ const markdownToTxtRenderer = {
   },
 
   heading({ tokens }) {
-    return this.parser?.parse(tokens) || '';
+    return this.parser?.parseInline(tokens) || '';
   },
 
   hr() {