doc: rockchip: Fix reference the wrong defconfig name of ROC-CC-RK3308

The defconfig file for ROC-CC-RK3308 is roc-cc-rk3308_defconfig.

Fixes: 7f08bfb7 ("doc: rockchip: Add documentation for rk3308 based
boards")

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>