Rename ringbuf-shm to ringbuf_shm

[ChunMinChang]

The ringbuf_shm link in README.md is not working since it's linked to
an inexistent ringbuf_shm folder. Renaming ringbuf-shm folder to
ringbuf_shm fixes the problem.

[jserv]

Why didn't change the top-level README.md instead?

[ChunMinChang]

I am comfortable with both approaches as long as they can fix the link.

The term in ringbuf-shm/README.md is named ringbuf_shm instead, and the _ is used in another folder's name (preempt_sched), so I guess the naming convention here prefers _.

I didn't file the PR in the first place because I'd like to keep the author's name on the files (in git blame and history). So renaming the folder's name in README.md rather than the folder's name itself is probably more in line with my need.

Feel free to let me know what works for you best.

[jserv]

The term in ringbuf-shm/README.md is named ringbuf_shm instead, and the _ is used in another folder's name (preempt_sched), so I guess the naming convention here prefers _.

I did not follow a consistent name scheme since these programs were written separately. The minimal change is fine to me.

[jserv]

Thank @ChunMinChang for contributing!