[ISSUE-10463] Add missing import in learning-to-rank tutorial (dmlc#10464)

* added 'sorted()' to qid, and added pandas import

* Update learning_to_rank.rst

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Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

Author: jpizagno

doc/tutorials/learning_to_rank.rst

@@ -71,6 +71,7 @@ Please note that, as of writing, there's no learning-to-rank interface in scikit
 
 .. code-block:: python
 
+  import pandas as pd
   df = pd.DataFrame(X, columns=[str(i) for i in range(X.shape[1])])
   df["qid"] = qid[sorted_idx]
   ranker.fit(df, y)  # No need to pass qid as a separate argument