changing a method to static

haystack/components/joiners/document_joiner.py

@@ -162,7 +162,8 @@ def run(self, documents: Variadic[List[Document]], top_k: Optional[int] = None):
 
         return {"documents": output_documents}
 
-    def _concatenate(self, document_lists: List[List[Document]]) -> List[Document]:
+    @staticmethod
+    def _concatenate(document_lists: List[List[Document]]) -> List[Document]:
         """
         Concatenate multiple lists of Documents and return only the Document with the highest score for duplicates.
         """