dt-bindings: display: st,stm32-dsi: Fix panel node name in example

With 'unevaluatedProperties' support enabled, the st,stm32-dsi binding
has a new warning:

Documentation/devicetree/bindings/display/st,stm32-dsi.example.dt.yaml: dsi@5a000000: Unevaluated properties are not allowed ('panel-dsi@0' was unexpected)

The documented child node name is 'panel', so update the example.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211221125145.1195234-1-robh@kernel.org