Fixup README typos

Fixes issue #4 on github[1]

[1]: https://github.com/couchbase/couchbase-ruby-model/pull/4

Change-Id: Idff3741eb4c4cf41b74cf9ddde861e3ffc092bcb
Reviewed-on: http://review.couchbase.org/24123
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>