fix missing underscore (#578)

Co-authored-by: zdh <935021934@qq.com>

Author: butterunderflow

scripts/extract-indices.mjs

@@ -22,13 +22,13 @@ const headers = options => (tree, file) => {
   tree.children.forEach(child => {
     if (child.type === "heading" && child.depth === 1) {
       if (child.children.length > 0) {
-        mainHeader = child.children[0].value;
+        mainHeader = child.children.map(element => element.value).join("");
       }
     }
     if (child.type === "heading" && child.depth === 2) {
       if (child.children.length > 0) {
         const id = child.data.id || "";
-        const name = child.children[0].value;
+        const name = child.children.map(element => element.value).join("");
         headers.push({ name, href: id });
       }
     }