Improve warnings for DES/3DES

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 2a2c039..9ae51c9 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -2428,7 +2428,7 @@
  *
  * PEM_PARSE uses DES/3DES for decrypting encrypted keys.
  *
- * \warning   DES is considered a weak cipher and its use constitutes a
+ * \warning   DES/3DES are considered weak ciphers and their use constitutes a
  *            security risk. We recommend considering stronger ciphers instead.
  */
 #define MBEDTLS_DES_C