bdev/raid: Fix more than a space between struct and name struct

This is just for readability.

Change-Id: Ic2a62ded78146c5b8736b6b7daebda1f3d5e3c47
Signed-off-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/454414


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>