rockchip: Clarify docs on SPI writing

We use every second block when creating a SPI image, so update the text to
say this explicitly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/doc/README.rockchip b/doc/README.rockchip
index 9542265..db5724e 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -262,7 +262,7 @@
    dd if=out.bin of=out.bin.pad bs=4M conv=sync
 
 This converts the SPL image to the required SPI format by adding the Rockchip
-header and skipping every 2KB block. Then the U-Boot image is written at
+header and skipping every second 2KB block. Then the U-Boot image is written at
 offset 128KB and the whole image is padded to 4MB which is the SPI flash size.
 The position of U-Boot is controlled with this setting in U-Boot: