Renamed b/brainf*ck.bf to b/brainf_ck.bf to avoid crossplatform problems

The filename `brainf*ck.bf` causes problems on Windows. I replaced `*` with underscore `_`.

The problem was quite interesting: how to checkout file with invalid filename in the current system?

Here is a solution:

    git show master:b/brainf*ck.bf > b/brainf_ck.bf