fix smaller stuff

- change filename option in example config file
- adapt default filename format for mangafox
- remove unnecessary newline

[skip ci]

docs/gallery-dl.conf

@@ -16,7 +16,7 @@
         {
             "user":
             {
-                "directory": ["{category}", "{artist-id}"]
+                "directory": ["{category}", "{user[id]}"]
             },
             "bookmark":
             {