Fix incorrect extend syntax

A syntax issue introduced in I9d2bba8d87643abfc626346427e90569d9ed7a3e.

Change-Id: I5d07e6f08ac2a8d49ae45888852746ab29453702
Partially-implements: bp/metadata-for-share-resources
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
diff --git a/manila_tempest_tests/tests/api/test_share_network_subnets.py b/manila_tempest_tests/tests/api/test_share_network_subnets.py
index a16dd3c..1ae5351 100644
--- a/manila_tempest_tests/tests/api/test_share_network_subnets.py
+++ b/manila_tempest_tests/tests/api/test_share_network_subnets.py
@@ -76,7 +76,7 @@
             "share_network_id", "mtu"
         ]
         if utils.is_microversion_ge(CONF.share.max_api_microversion, '2.78'):
-            keys.extend('metadata')
+            keys.extend(['metadata'])
 
         # Default subnet was created during share network creation
         self.assertIsNone(default_subnet['availability_zone'])