Replace tls1_3 with tls13

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/tests/scripts/generate_tls13_compat_tests.py b/tests/scripts/generate_tls13_compat_tests.py
index 5f6eead..07013ae 100755
--- a/tests/scripts/generate_tls13_compat_tests.py
+++ b/tests/scripts/generate_tls13_compat_tests.py
@@ -243,7 +243,7 @@
     def cmd(self):
         ret = ['$P_CLI']
         ret += ['server_addr=127.0.0.1', 'server_port=$SRV_PORT',
-                'debug_level=4', 'force_version=tls1_3']
+                'debug_level=4', 'force_version=tls13']
         ret += ['ca_file={cafile}'.format(
             cafile=CERTIFICATES[self._sig_algs[0]].cafile)]