README: Update 'r-gaia-cs' -> 'rgaiacs'

I thought the hyphenated form was current when I used it in 0d8da61e
(README.md: Add local maintainer listing, 2015-04-15, #2), but GitHub
no longer recognizes the hyphenated form.  The un-hyphenated form
seems to date back to 2012-03-06 [1], so maybe I was just confused ;).
Anyhow, fixing to use the current username.

[1]: https://github.com/rgaiacs