docs: fix the command to compile BL31 on Tegra

This patch fixes the command line used to compile BL31 on
Tegra platforms.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

docs/plat/nvidia-tegra.md

@@ -57,7 +57,7 @@ without changing any makefiles.
 Preparing the BL31 image to run on Tegra SoCs
 ===================================================
 'CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf- make PLAT=tegra \
-TARGET_SOC=<target-soc e.g. t210|t132> SPD=<dispatcher e.g. tlkd> all'
+TARGET_SOC=<target-soc e.g. t210|t132> SPD=<dispatcher e.g. tlkd> bl31'
 
 Platforms wanting to use different TZDRAM_BASE, can add 'TZDRAM_BASE=<value>'
 to the build command line.