Update GraphCacheList.java

Author: gkorland

src/main/java/com/redislabs/redisgraph/impl/GraphCacheList.java

@@ -18,7 +18,7 @@ public class GraphCacheList {
     private final String graphId;
     private final String procedure;
     private final RedisGraph redisGraph;
-    List<String>  data = new CopyOnWriteArrayList<>();
+    private final List<String>  data = new CopyOnWriteArrayList<>();