[maskedtensor] fix docs formatting

[george-qi]

Stack from ghstack (oldest at bottom):

• -> [maskedtensor] fix docs formatting #87387

[pytorch-bot]

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❌ 3 Failures

As of commit de57e95:

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• macos-12-py3-x86-64 / test (default, 1, 2, macos-12)
• macos-12-py3-x86-64 / test (default, 2, 2, macos-12)
• macos-12-py3-x86-64 / test (functorch, 1, 1, macos-12)

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[george-qi]

@pytorchbot merge -f "Trivial docs typo fix"

[pytorchmergebot]

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[github-actions]

Hey @george-qi.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.