[llvm] [FileCheck][Docs] Fix regex for FileCheck variable names (PR #97301)

[Anton Lydike via llvm-commits]

AntonLydike wrote:

@RoboTux sorry, new to all of this, the windows build failure seems not related to this change, can I just merge or should I rebase and re-run CI?

https://github.com/llvm/llvm-project/pull/97301