Update README to match master->main branch change (facebookresearch#3922

)

Summary:
Loading xlmr doesn't work because trying to pull from master branch, which has been changed to main.

Pull Request resolved: facebookresearch#3922

Reviewed By: erichan1

Differential Revision: D31415771

Pulled By: dianaml0

fbshipit-source-id: a4b1b533b5596a4d3556c97c2f15847e444aeea9

examples/xlmr/README.md

@@ -71,7 +71,7 @@ Model | average | en | es | de | ar | hi | vi | zh
 ##### Load XLM-R from torch.hub (PyTorch >= 1.1):
 ```python
 import torch
-xlmr = torch.hub.load('pytorch/fairseq', 'xlmr.large')
+xlmr = torch.hub.load('pytorch/fairseq:main', 'xlmr.large')
 xlmr.eval()  # disable dropout (or leave in train mode to finetune)
 ```