REST test: Fixed index-seal test

Fixed bad YAML, and changed to wait for yellow instead of green, because
REST tests usually run on a single node

rest-api-spec/test/indices.seal/10_basic.yaml

@@ -1,16 +1,16 @@
 ---
 "Index seal rest test":
-- do:
-   indices.create:
-      index: testing
+    - do:
+       indices.create:
+          index: testing
 
-- do:
-   cluster.health:
-      wait_for_status: green
-- do:
-   indices.seal:
-      index: testing
-- do:
-   indices.stats: {level: shards}
+    - do:
+       cluster.health:
+          wait_for_status: yellow
+    - do:
+       indices.seal:
+          index: testing
+    - do:
+       indices.stats: {level: shards}
 
-- is_true: indices.testing.shards.0.0.commit.user_data.sync_id
+    - is_true: indices.testing.shards.0.0.commit.user_data.sync_id