doc: misc-device: add uacce to toctree(index)

This fixes:

Documentation/misc-devices/uacce.rst: WARNING: document isn't included in any toctree
Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: https://lore.kernel.org/r/20200430225828.114033-1-vitor@massaru.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/misc-devices/index.rst

@@ -21,4 +21,5 @@ fit into other categories.
    lis3lv02d
    max6875
    mic/index
+   uacce
    xilinx_sdfec