Fix smoke test vm example in README

Author: alexjfisher

tests/smoke/README.md

@@ -19,7 +19,7 @@ vm = {
     is_windows_image  = false
     vmrp              = "fill"
     dc                = "fill" 
-    datastore_cluster = "fill"
+    datastore         = "fill"
     vmfolder          = "fill"
     vmgateway         = "10.13.13.1"
     dns_servers       = ["1.1.1.1"]