doc: add missing caption for extlinks

Starting with Sphinx 6.0, extlinks caption with %s will be required.
Sphinx 5.x already warns about this requirements, so fix it.

(cherry picked from commit 2a93e8cc44bd383bccebdbb5265a013e8f207d58)

Original-Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
GitOrigin-RevId: 2a93e8cc44bd383bccebdbb5265a013e8f207d58
Change-Id: I4b77781f8089010d7f0846f3530a1c56e275e196
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/3829406
Tested-by: CopyBot Service Account <copybot.service@gmail.com>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
1 file changed