Fix typo in alarm backend name

It should be postgresq"l" instead of postgresq.

Change-Id: I0b8ab464bdf0227a8efb29a90087adc900d60225
diff --git a/telemetry_tempest_plugin/config.py b/telemetry_tempest_plugin/config.py
index ad4ea85..c8f757b 100644
--- a/telemetry_tempest_plugin/config.py
+++ b/telemetry_tempest_plugin/config.py
@@ -93,7 +93,7 @@
                 help="Backend store used to store metrics"),
     cfg.StrOpt('alarm_backend',
                default='mysql',
-               choices=['mysql', 'postgresq'],
+               choices=['mysql', 'postgresql'],
                help="Database used by the aodh service"),
 ]