[docs] XLnet reference link bug in description of past_index Parameter of TrainingArguments 
XLnet reference link bug in description of past_index Parameter of TrainingArguments 
link to the doc: https://huggingface.co/transformers/main_classes/trainer.html#transformers.TrainingArguments

**current description:** Some models like TransformerXL or **:doc`XLNet <../model_doc/xlnet>`** can make use of the past hidden states for their predictions. If this argument is set to a positive int, the Trainer will use the corresponding output (usually index 2) as the past state and feed it to the model at the next training step under the keyword argument mems.

**expected description:** Some models like TransformerXL or **XLNet** can make use of the past hidden states for their predictions. If this argument is set to a positive int, the Trainer will use the corresponding output (usually index 2) as the past state and feed it to the model at the next training step under the keyword argument mems.

## Environment info
Not required
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### Who can help
@sgugger
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Thanks for flagging! Should be fixed by the PR linked above!