Fix typo in FP16 accumulation section

[jiahy0825]

Fix typo error

[pytorch-bot]

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[linux-foundation-easycla]

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This PR needs a release notes: label

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@pytorchbot label "topic: not user facing"

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[jiahy0825]

@pytorchbot label "release notes:"

[pytorch-bot]

Didn't find following labels among repository labels: release notes:2.9