[llvm] [llvm][llvm-as] Fix file input extension description (PR #108295)

[David Spickett via llvm-commits]

https://github.com/DavidSpickett approved this pull request.

Given that `--help` starts with:
```
OVERVIEW: llvm .ll -> .bc assembler
```
This looks correct to me, good work spotting it.

Do you have the ability to merge the PR or should I do it for you?

https://github.com/llvm/llvm-project/pull/108295