doc: usage: Fix command in fdt overlay apply sequence

Literally adhering to the docs gave this wrong output:

    U-Boot> setenv fdtaddr 0x87f00000
    U-Boot> fdtaddr $fdtaddr
    Unknown command 'fdtaddr' - try 'help'

Fixes: d80162cf ("doc: Document how to apply fdt overlays")
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>