Update Command for Build Docs

CMAKE_TOOLCHAIN_FILE has been replaced with TFM-TOOLCHAIN_FILE.

Change-Id: Iae867801ebae4ba1392f1299d8a852c6167ead68
Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
diff --git a/build-docs.sh b/build-docs.sh
index a1f34e0..77fb000 100755
--- a/build-docs.sh
+++ b/build-docs.sh
@@ -20,7 +20,7 @@
 cd ../trusted-firmware-m/build
 
 cmake -S .. -B . -DTFM_PLATFORM=mps2/an521 \
-                 -DCMAKE_TOOLCHAIN_FILE=toolchain_GNUARM.cmake \
+                 -DTFM_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake \
                  -DMBEDCRYPTO_PATH=../../mbedtls \
                  -DTFM_TEST_REPO_PATH=../../tf-m-tests \
                  -DMCUBOOT_PATH=../../mcuboot