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

[Amr Hesham via llvm-commits]

AmrDeveloper wrote:

> 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?

Thank you, I can merge

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