Fix typo to enable sg-core service in tempest conf

The telemetry_tempest_plugin.scenario.test_telemetry_integration_prometheus test is currently being skipped, because sg-core isn't configured correctly as available.

See https://storage.gra.cloud.ovh.net/v1/AUTH_dcaab5e32b234d56b626f72581e3644c/zuul_opendev_logs_57b/908258/2/gate/telemetry-dsvm-integration/57bd243/testr_results.html where it's skipped.

Change-Id: I02fba4a64afb5765f47d48283b17d2f54d951bb7
diff --git a/.zuul.yaml b/.zuul.yaml
index 834ea1e..647dd54 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -52,7 +52,7 @@
         test-config:
           $TEMPEST_CONFIG:
             service_available:
-              sg-core: True
+              sg_core: True
             telemetry_services:
               metric_backends: gnocchi,prometheus
             telemetry: