Fix typo in cmake file for IAR

Commit 5d8adab9838d8ee6b18edd6a42e75fd8fd191 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>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6840295..b11215d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -245,7 +245,7 @@
     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)