Update end_to_end_fine_tuning.rst (#1206)

Removed duplicate parameter in DPO configuration which otherwise leads to an error.

Trace for visibility:
```
> fairseq2 lm instruction_finetune $OUTPUT_DIR --config-file $CONFIG_FILE
(...)
 File "/Users/aerben/miniconda3/envs/fairseq2/lib/python3.12/site-packages/ruamel/yaml/constructor.py", line 281, in check_mapping_key
                                 raise DuplicateKeyError(*args)
                             ruamel.yaml.constructor.DuplicateKeyError: while constructing a mapping
                               in "temp.config", line 3, column 9
                             found duplicate key "max_seq_len" with value "4096" (original value: "4096")
                               in "temp.config", line 6, column 9
```

Author: cirquit

doc/source/tutorials/end_to_end_fine_tuning.rst

@@ -146,7 +146,6 @@ Read more about this recipe in :ref:`tutorial-preference-optimization`.
                 path: /datasets/facebook/fairseq2-lm-gsm8k/sft
                 max_seq_len: 4096
                 max_num_tokens: 4096
-                max_seq_len: 4096
         model:
             _set_:
                 name: llama3_2_1b