Fix typo in cmake file for IAR

Commit 5d8adab introduced a typo in the flag for the IAR compiler "--warnings_are_errors" is the correct flag

Signed-off-by: Robin Kastberg <robin.kastberg@iar.com>

CMakeLists.txt

@@ -242,7 +242,7 @@ if(MBEDTLS_FATAL_WARNINGS)
     endif(CMAKE_COMPILER_IS_CLANG OR CMAKE_COMPILER_IS_GNU)
 
     if (CMAKE_COMPILER_IS_IAR)
-        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warning_are_errors")
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warnings_are_errors")
     endif(CMAKE_COMPILER_IS_IAR)
 endif(MBEDTLS_FATAL_WARNINGS)