doc: Fix reference to Rock pro when Rock 2 is meant

The Radxa Rock pro board is rk3188 based and thus won't work with U-Boot
built for RK3288. Change the documentation to refer to the intended
board, the Radxa Rock 2, which is an RK3288-based design very similar to
the firefly

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
1 file changed