Docs: Fix typo in build instructions

The build instructions for the PSA Developer API tests
contain a typo in the cmake string to use.

Change-Id: I2655d74ef72e08e1a729d98a8f3e668f7e342934
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index 6c89699..b817353 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -140,7 +140,7 @@
     cd trusted-firmware-m
     mkdir cmake_psa_test
     cd cmake_psa_test
-    cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigPsaApiTest.cmake` -DPSA_API_TEST_CRYPTO -DTARGET_PLATFORM=AN521 -DCOMPILER=ARMCLANG ../
+    cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigPsaApiTest.cmake` -DPSA_API_TEST_CRYPTO=ON -DTARGET_PLATFORM=AN521 -DCOMPILER=ARMCLANG ../
     cmake --build ./ -- install
 
 Location of build artifacts