fix linting of MetaFieldGroupingRanker (#8570)

Author: anakin87

haystack/components/rankers/meta_field_grouping_ranker.py

@@ -106,9 +106,8 @@ def run(self, documents: List[Document]) -> Dict[str, Any]:
                 no_group_docs.append(doc)
 
         ordered_docs = []
-        for group in document_groups:
-            for subgroup in document_groups[group]:
-                docs = document_groups[group][subgroup]
+        for subgroups in document_groups.values():
+            for docs in subgroups.values():
                 if self.sort_docs_by:
                     docs.sort(key=lambda d: d.meta.get(cast(str, self.sort_docs_by), float("inf")))
                 ordered_docs.extend(docs)