[llvm] [RISCV] Fix typo in UserGuides.rst. NFC (PR #113861)

[Alex Bradbury via llvm-commits]

asb wrote:

> > Do feel free to just directly commit simple fixes like this (and good catch!).
> 
> Do you mean commit without a PR?

Yes, this is allowed in cases like this per the code review policy https://llvm.org/docs/CodeReview.html#must-code-be-reviewed-prior-to-being-committed

https://github.com/llvm/llvm-project/pull/113861