docs/system/arm: Fix for rename of type "xlnx.bbram-ctrl"

Fixes: b65b4b7 (xlnx-bbram: hw/nvram: Use dot in device type name)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
[thuth: Use longhand syntax to avoid problems with the "." in the name]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20231117114457.177308-2-thuth@redhat.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/system/arm/xlnx-versal-virt.rst

@@ -194,7 +194,7 @@ To use a different index value, N, from default of 0, add:
 
 .. code-block:: bash
 
-  -global xlnx,bbram-ctrl.drive-index=N
+  -global driver=xlnx.bbram-ctrl,property=drive-index,value=N
 
 eFUSE File Backend
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