filters/flatten: fix input documentation (#84477)

Flatten input is a list, not a dictionary.

lib/ansible/plugins/filter/flatten.yml

@@ -7,8 +7,9 @@ DOCUMENTATION:
   positional: _input, levels, skip_nulls
   options:
     _input:
-      description: First dictionary to combine.
+      description: List to flatten.
-      type: dict
+      type: list
+      elements: any
       required: true
     levels:
       description: Number of recursive list depths to flatten.