rcar-gen3_setup: Fix wrong u-boot environment command

The right command to reset the default environment is `env`, not
`setenv`. Fix this.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

content/reference_hardware/rcar-gen3_setup.md

@@ -127,7 +127,7 @@ and reset the board. To do this on the boards currently available to us:
 It is recommended to reset the U-Boot environment after flashing new version of bootloader.
 Stop in U-Boot prompt and execute commands:
 
-    setenv default -a
+    env default -a
     saveenv
 
 After reset the board should now boot Apertis from SD card automatically.