fixed attribute name

Author: jsteemann

src/main/java/com/arangodb/entity/BaseDocument.java

@@ -14,7 +14,7 @@ public class BaseDocument extends BaseEntity implements DocumentHolder {
 
 	public static final String KEY = "_key";
 
-	public static final String ID = "_ID";
+	public static final String ID = "_id";
 
 	public static final String FROM = "_from";