llvm-ar `N` argument docs: final sentence is incomplete

The last sentence of the docs for llvm-ar `N` starts, but doesn't finish.

> ```
> If *count* is not supplied, the operation fails.*count* cannot be
> ```

https://github.com/llvm/llvm-project/blob/llvmorg-21.1.2/llvm/docs/CommandGuide/llvm-ar.rst?plain=1#L184-L191

Additionally, [on the website](https://llvm.org/docs/CommandGuide/llvm-ar.html) "count" is not rendered with italics:

Open llvm/docs/CommandGuide/llvm-ar.rst around the `N` argument at lines 184–191 and compare it with the rendered website version. Complete the unfinished sentence and correct the `count` markup, then verify that the source and generated documentation display the intended wording and italics.

Written by the indexing model from the issue text.