markdownlint: enable rule MD018

MD018 - No space after hash on atx style header

Signed-off-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Change-Id: Ifece1ef5e33093ed34b0d7f75b4cbe3ac37ca37b
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9036


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>