[Bugfix:Autograding] missing comma in scheme metadata file (#74)

Co-authored-by: Barb Cutler <Barb Cutler>

Author: bmcutler

dockerfiles/scheme/metadata.json

@@ -1,4 +1,4 @@
 {
-    "latest" : "8.18"
+    "latestTag" : "8.18",
     "pushLatest": true
 }