DOC fix momentum equation for nesterov
Fix https://github.com/pytorch/pytorch/issues/72395
This is a small fix in the doc for an indice in this equation:
I think teh indice should not be `t-1` but `t`. This is coherent with [the implementation)[https://github.com/pytorch/pytorch/blob/master/torch/optim/sgd.py#L236] and with what is done for instance in [keras](https://www.tensorflow.org/api_docs/python/tf/keras/optimizers/SGD).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76639
Approved by: https://github.com/albanD
