[llvm] [ADT] Fix a comment typo (PR #65497)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Learning as well...
```
% gh pr review 65497
? What kind of review do you want to give? Approve
? Review body <Received>
? Submit? Yes
✓ Approved pull request #65497
```

https://github.com/llvm/llvm-project/pull/65497