fix(rockchip): fix documentation in how build bl31 in AARCH64

Rockchip Aarch64 SoCs expect TF-A's BL31

Change-Id: Ie74be32e2bd24c4de38990791b4a03d2b7695b4d
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
diff --git a/docs/plat/rockchip.rst b/docs/plat/rockchip.rst
index b7c43fb..01cf176 100644
--- a/docs/plat/rockchip.rst
+++ b/docs/plat/rockchip.rst
@@ -35,7 +35,7 @@
 
 For AARCH64 architectures the build command looks like
 
-    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399 bl32
+    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399 bl31
 
 while AARCH32 needs a slightly different command