[I] [C++] Fix PartitionSort example. [arrow]

[via GitHub]

Light-City opened a new issue, #44677:
URL: https://github.com/apache/arrow/issues/44677

   ### Describe the bug, including details regarding any error messages, 
version, and platform.
   
   ```
     /// For example:
     ///
     /// in_arr: [5, 7, 2, 3, 5, 4]
     /// num_prtns: 3
     /// prtn_id_impl: [&in_arr] (int row_id) { return in_arr[row_id] / 3; }
     /// output_pos_impl: [&out_arr] (int row_id, int pos) { out_arr[pos] = 
row_id; }
     ///
   ```
   
   output should be:
   
   ```
     /// out_arr: [2, 0, 3, 4, 5, 1]
     /// prtn_ranges: [0, 1, 5, 6]
   ```
   but now is:
   
   ```
     /// out_arr: [2, 5, 3, 5, 4, 7]
     /// prtn_ranges: [0, 1, 5, 6]
   ```
   
   out_arr value is row id not actual value.
   
   ### Component(s)
   
   C++


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