fix typo from https://github.com/EleutherAI/gpt-neox/pull/1244/files#…

…diff-383134de6f3512484e20625419bd5fb6b1675a922f47aeb1a6bd3cff6185a754R126

megatron/data/gpt2_dataset.py

@@ -123,7 +123,7 @@ def __getitem__(self, idx):
                         samples.append(
                             dataset.get(
                                 self.doc_idx[doc_index_f],
-                                offset=offset_l,
+                                offset=offset_f,
                                 length=offset_l - offset_f + 1,
                             )
                         )