docs: fix typo

Author: ar90n

README.md

@@ -84,6 +84,6 @@ trainer.fit(model, train_dataloaders=DataLoader(RandomDataset(32, 64), batch_siz
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 [issues-url]: https://github.com/ar90n/pytorch-lightning-sam-callback/issues
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-[license-url]: https://github.com/ar90n/pytorch-lightning-sam-callback/blob/master/LICENSE
+[license-url]: https://github.com/ar90n/pytorch-lightning-sam-callback/blob/main/LICENSE
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