[doc] Fux learning_to_rank.rst (dmlc#9381)

just adding one missing bracket

Author: cassiasamp

doc/tutorials/learning_to_rank.rst

@@ -70,7 +70,7 @@ Please note that, as of writing, there's no learning-to-rank interface in scikit
 
 .. code-block:: python
 
-  df = pd.DataFrame(X, columns=[str(i) for i in range(X.shape[1]))
+  df = pd.DataFrame(X, columns=[str(i) for i in range(X.shape[1])])
   df["qid"] = qid
   ranker.fit(df, y)  # No need to pass qid as a separate argument