Fix documentation for using slave shards

Not sure of the history, but using the single read-only slave, single master example in the docs, I had to use a key named `slave` instead of `read_only`.

Using sequel 4.10.0, mysql2 0.3.16, ruby 2.1.0 and a threaded connection pool.

I didn't see if this was necessary for the other examples, but I can update those as well if appropriate.

Thanks for the hard work!