doc: board: siemens: iot2050: Update build env vars

ATF is now called BL31, and OP-TEE since 3.21 suggests to use
tee-raw.bin instead of (the still identical) tee-pager_v2.bin.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
diff --git a/doc/board/siemens/iot2050.rst b/doc/board/siemens/iot2050.rst
index 0df11a9..ee3c5c9 100644
--- a/doc/board/siemens/iot2050.rst
+++ b/doc/board/siemens/iot2050.rst
@@ -66,8 +66,8 @@
 
 .. code-block:: text
 
- $ export ATF=/path/to/bl31.bin
- $ export TEE=/path/to/tee-pager_v2.bin
+ $ export BL31=/path/to/bl31.bin
+ $ export TEE=/path/to/tee-raw.bin
  $ make iot2050_defconfig
 
  $ make