doc: userspace-api: properly format ToC headings

"class:: toc-title" was a workaround for older Sphinx versions that are
no longer supported.

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Cc: Rui Li <me@lirui.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Wu XiangCheng <bobwxc@email.cn>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-3-vegard.nossum@oracle.com

Documentation/translations/zh_CN/userspace-api/index.rst

@@ -17,11 +17,8 @@ Linux 内核用户空间API指南
 在代码树中仍然可以找到有关用户空间的部分信息。这个手册意在成为这些信息
 聚集的地方。
 
-.. class:: toc-title
-
-	   目录
-
 .. toctree::
+   :caption: 目录
    :maxdepth: 2
 
    no_new_privs

Documentation/userspace-api/index.rst

@@ -9,11 +9,8 @@ While much of the kernel's user-space API is documented elsewhere
 also be found in the kernel tree itself.  This manual is intended to be the
 place where this information is gathered.
 
-.. class:: toc-title
-
-	   Table of contents
-
 .. toctree::
+   :caption: Table of contents
    :maxdepth: 2
 
    no_new_privs