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 <[email protected]>
Cc: Yanteng Si <[email protected]>
Cc: Wu XiangCheng <[email protected]>
Signed-off-by: Vegard Nossum <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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