Update alert message for parsing PSK hint

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 6ea7865..fb33b54 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -3006,7 +3006,7 @@
             mbedtls_ssl_send_alert_message(
                 ssl,
                 MBEDTLS_SSL_ALERT_LEVEL_FATAL,
-                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+                MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_DECODE_ERROR );
         }
     } /* FALLTROUGH */