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>

doc/README.rockchip

@@ -39,7 +39,7 @@ Building
 At present three RK3288 boards are supported:
 
    - Firefly RK3288 - use firefly-rk3288 configuration
-   - Radxa Rock Pro - also uses firefly-rk3288 configuration
+   - Radxa Rock 2 - also uses firefly-rk3288 configuration
    - Haier Chromebook - use chromebook_jerry configuration
 
 For example:
@@ -48,8 +48,8 @@ For example:
 
 (or you can use another cross compiler if you prefer)
 
-Note that the Radxa Rock Pro uses the Firefly configuration for now as
-device tree files are not yet available for the Rock Pro. Clearly the two
+Note that the Radxa Rock 2 uses the Firefly configuration for now as
+device tree files are not yet available for the Rock 2. Clearly the two
 have hardware differences, so this approach will break down as more drivers
 are added.