[docs] add parent>child example (#4621)

docs/gallery-dl-example.conf

@@ -176,16 +176,15 @@
 
         "imgur":
         {
-            "#": "use different directory and filename formats when coming from a reddit post",
-            "directory":
-            {
-                "'_reddit' in locals()": []
-            },
-            "filename":
-            {
-                "'_reddit' in locals()": "{_reddit[id]} {id}.{extension}",
-                ""                     : "{id}.{extension}"
-            }
+            "#": "general imgur settings",
+            "filename": "{id}.{extension}"
+        },
+
+        "reddit>imgur":
+        {
+            "#": "special settings for imgur URLs found in reddit posts",
+            "directory": [],
+            "filename": "{_reddit[id]} {_reddit[title]} {id}.{extension}"
         },
 
         "tumblr":